An adverse event (AE) was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study drug, whether or not considered related to the study drug. An SAE was defined as any untoward medical occurrence (whether considered to be related to study drug or not) that at any dose: resulted in death; or was life-threatening; or required inpatient hospitalization or prolongation of existing hospitalization; or resulted in persistent or significant disability/incapacity; or was a congenital anomaly/birth defect; or was an important medical event. An AESI was an AE (serious or nonserious) of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and immediate notification by the Investigator to the Sponsor was required. The TEAEs were defined as events that were newly reported or reported to worsen in severity after the start of study drug up to 5 days post last dose of the study drug.

The Safety population included all enrolled participants who had received at least 1 dose of study drug, regardless of the amount of study drug administered.

Post-treatment annualized growth velocity (AGV) was calculated from height measurements collected during the study. AGV was calculated as: AGV (centimeter per year)= (Height at Date 2 - Height at Date 1) divided by interval length in days x 365.25, where interval length in days was calculated as (Date 2 - Date 1+1). Z scores were calculated using LMS methods (Box Cox power L, Median M and Variation S), where appropriate: If L≠ 0: Z= [(AGV/M)^L-1]/(LxS); If L= 0: Z= ln(AGV/M)/S. A Z score of 0 represents the mean AGV of the reference achondroplasia (ACH) population. For AGV Z scores in children with ACH: Higher Z scores represent better outcomes, meaning faster linear growth relative to age and sex matched reference populations. Lower Z scores represent worse outcomes, indicating slower growth compared to reference norms. Baseline value was defined as last available pre dose height measurement meeting statistical analysis plan specified time interval definitions.

The Intent-to-treat (ITT) analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The AGV was defined as follows: (Height at Date 2 - height at Date 1) divided by interval length in days x 365.25, where the interval length in days was calculated as (Date 2 - Date 1 +1) days. The post-treatment AGV was calculated from height measurements collected during the study. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants.

Post-treatment AGV was calculated from height measurements collected during the study. AGV was calculated as: AGV (centimeter per year)= (Height at Date 2 - Height at Date 1) divided by interval length in days x 365.25, where interval length in days was calculated as (Date 2 - Date 1+1). Z scores were calculated using LMS methods (Box Cox power L, Median M and Variation S), where appropriate: If L≠ 0: Z= [(AGV/M)^L-1]/(LxS); If L= 0: Z= ln(AGV/M)/S. A Z score of 0 represents the mean AGV of the reference population (ACH). For AGV Z scores in children with ACH: Higher Z scores represent better outcomes, meaning faster linear growth relative to age and sex matched reference populations. Lower Z scores represent worse outcomes, indicating slower growth compared to reference norms. Baseline value was defined as last available pre dose height measurement meeting statistical analysis plan specified time interval definitions.

The ITT analysis set included all enrolled participants.

The upper to lower body segment ratio was calculated by (standing height or total length - lower body segment length) divided by lower body segment length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The upper to lower extremity ratio was calculated by (upper arm + forearm length) divided by (upper leg + lower leg length). The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The sitting to standing height ratio was calculated by sitting height divided by standing height, or crown to rump length divided by total length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The arm span to height ratio was calculated by arm span divided by standing height, or arm span divided by total length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The upper arm to forearm length ratio was calculated by upper arm length divided by forearm length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The upper leg to lower leg length ratio was calculated by upper leg length divided by lower leg length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The head circumference to height ratio was calculated by head circumference divided by standing height, or head circumference divided by total length. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The magnetic resonance imaging was planned to be collected to measure foramen magnum and other brainstem, skull, and spine morphometric parameters in Stage 2 participants. The baseline value was defined as the last available value before the first dose of study drug.

Stage 2 of the study was not yet initiated prior to the early termination of the trial.

The pediatric quality of life (PedsQL) Generic Core Scales assesses pediatric health related quality of life (HRQOL) across functioning domains. Items use a 5-point scale (0=Never to 4=Almost Always). Responses are reverse scored and linearly transformed to a 0-100 scale (0→100, 1→75, 2→50, 3→25, 4→0), where higher scores indicate better quality of life (fewer problems). Domain scores for Social, Emotional, and School Functioning are calculated from the transformed item scores. If more than 50% of items in a domain are missing, the domain score is not computed. If at least 50% are completed, the domain score equals the mean of the non missing transformed item scores. All domain scores range from 0 (almost always) to 100 (never), with higher scores indicate better HRQOL. Baseline was defined as the last available value prior to the first dose of study drug.

The ITT analysis set included all enrolled participants. Due to early termination of the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The PedsQL core scale was a brief, standardized, generic instrument that was used to measure HRQOL in various pediatric conditions. Psychosocial health summary scores were the meaning of item scores in emotional, social, and school functioning scales; and Physical health summary scores were the meaning of item scores in physical scale. Total score was the mean of all the item scores in all scales. Psychosocial and physical health summary scores and total scores each ranged from 0 (almost always) to 100 (never). Higher scores indicate better HRQOL. For participants aged 5 to 7, only parents completed the questionnaire. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. Due to early termination of the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The screening tool for everyday mobility and symptoms (STEMS) was completed by clinicians for participants 5 years of age and older. The STEMS captures mobility, use of mobility aides, and impact of pain and/or fatigue at the end of the day across 3 environments: home, school/work and community. Mobility aides were recorded using numeric 5-point rating scale, score ranged from 1 to 5, where 1= independent on all surfaces including stairs, 2= use of sticks, 3= use of crutches, 4= use of wheeled walking device, and 5= use of wheelchair or mobility scooter. Lower score indicated the better outcome. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The STEMS was completed by clinicians for participants 5 years of age and older. The STEMS captures mobility, use of mobility aides, and impact of pain and/or fatigue at the end of the day across 3 environments: home, school/work and community. Participant reported symptoms were recorded using letters where A= no pain or fatigue, B1= pain only, B2= fatigue and C= pain and fatigue. This was completed for each of the 3 environments resulting in 3 scores, whereby the numeric rating reflects the use of mobility aides and the letter reflects the symptoms. The baseline value was defined as the last available value before the first dose of study drug. Only participants with a Baseline response of "A" were included in the "A to A," "A to B1," and "A to missing" categories at each time point. Similarly, only participants with a Baseline response of "B1" were included in the "B1 to A" and "B1 to missing" categories at each time point.

The ITT analysis set included all enrolled participants. For this outcome, the number of participants analyzed in each row reflects only the subset of participants who had non-missing STEMS symptom ratings at both baseline and the specific post baseline timepoint (Week 26 or Week 52) for the corresponding environment (home, school/work, community). The overall number of participants analyzed for the measure represents all participants with any evaluable STEMS data at the specified timepoint.

The PedsQL Multidimensional Fatigue Scale measures fatigue across 3 subscales:General Fatigue, Sleep/Rest Fatigue and Cognitive Fatigue. Items are rated on a 5 point scale (0=Never to 4=Almost Always). Responses are reverse scored and linearly transformed to a 0-100 scale (0→100, 1→75, 2→50, 3→25, 4→0), with higher scores indicate less fatigue and better functioning. Subscale scores are calculated as mean of transformed item scores within each subscale. A Total Score was calculated as mean of all transformed items across subscales. If more than 50% of items are missing within a subscale or across total scale, corresponding score is not computed. If at least 50% are completed, scores are calculated as mean of non missing transformed items. All scores range from 0 (no pain) to 100 (severe pain), with higher scores indicate less fatigue. Baseline was defined as last available value prior to first dose of study drug. For participants aged 5-7 years, only parent proxy report was completed.

The ITT analysis set included all enrolled participants. Due to early termination of the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

The PedsQL PPQ assesses current pain and worst pain intensity in the last week using a visual analog scale as well as pain location using a body map. Current and worst pain were scored separately, and scoring was based on the line length to the nearest 0.5 centimeter (5 millimeter). The score ranged from 0 (no pain) to 100 (severe pain), where higher scores indicated greater levels of pain intensity. The PedsQL PPQ was only administered in participants ages 5 years of age and older using both self-report (ages 8 and older) and parent proxy report (ages 5 and older) forms in all countries. For participants aged 5 to 7, only parents completed the questionnaire. The baseline value was defined as the last available value before the first dose of study drug.

The ITT analysis set included all enrolled participants. Due to early termination of the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

For Stage 2 participants, achievement of developmental milestones was planned to be assessed using the ACH developmental recording form. This form allows developmental milestones to be recorded and was not a formalized screening tool or assessment. The form measures the domains of gross motor function, fine motor function, communication and feeding via participant reports to the Investigator. The form depicts the cumulative percentage distributions for children with ACH for each item as well as norms for reference based on the Denver II test, where available. Data collected via this form was planned to be evaluated descriptively to explore the age at which participants in this study achieve developmental milestones. The baseline value was defined as the last available value before the first dose of study drug.

Stage 2 of the study was not yet initiated prior to the early termination of the trial.

Blood samples are collected to determine plasma concentration of SAR442501.

The Pharmacokinetic (PK) analysis set included all participants from the safety population with at least 1 post-baseline PK result with adequate documentation of dosing and sampling dates and times. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Blood samples are collected to determine Cmax of SAR442501. The Cmax of SAR442501 was calculated using non-compartmental method.

The PK analysis set included all participants from the safety population with at least 1 post-baseline PK result with adequate documentation of dosing and sampling dates and times.

Blood samples are collected to determine tmax of SAR442501. The tmax of SAR442501 was calculated using non-compartmental method.

The PK analysis set included all participants from the safety population with at least 1 post-baseline PK result with adequate documentation of dosing and sampling dates and times.

Blood samples are collected to determine AUC0-tau of SAR442501. The AUC0-tau of SAR442501 was calculated using non-compartmental method.

The PK analysis set included all participants from the safety population with at least 1 post-baseline PK result with adequate documentation of dosing and sampling dates and times.

Blood samples are collected to determine Ctrough of SAR442501. The Ctrough of SAR442501 was calculated using non-compartmental method.

The PK analysis set included all participants from the safety population with at least 1 post-baseline PK result with adequate documentation of dosing and sampling dates and times. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Serum samples were collected to assess the change in CXM level with study drug. The CXM was also named PRO-C10 because the biomarker assay used was aimed to target the recognition of the C terminus of the NC1 domain of type X collagen. The baseline value was defined as the last available value before the first dose of study drug. Change from baseline in CXM level has been reported.

The pharmacodynamic (PD) analysis set for CXM included participants with at least 1 post-baseline CXM assessment. Results for participants with CXM data available at Week 26 has been reported.

Serum samples were collected to assess the change in osteocalcin level with study drug. The baseline value was defined as the last available value before the first dose of study drug.

The PD analysis set included all participants from the safety population with at least 1 post-baseline PD parameter assessed. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Serum samples were collected to assess the change in bone-specific alkaline phosphatase level with study drug. The baseline value was defined as the last available value before the first dose of study drug.

The PD analysis set included all participants from the safety population with at least 1 post-baseline PD parameter assessed. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Serum samples were collected to assess the change in P1NP level with study drug. The baseline value was defined as the last available value before the first dose of study drug.

The PD analysis set included all participants from the safety population with at least 1 post-baseline PD parameter assessed. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Serum samples were collected to assess the change in CTX level with study drug. The baseline value was defined as the last available value before the first dose of study drug.

The PD analysis set included all participants from the safety population with at least 1 post-baseline PD parameter assessed. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

Serum samples were collected to assess the antibodies to SAR442501. Samples were screened and then confirmed for antibodies binding to SAR442501 and the titer of confirmed positive samples were reported. Positive ADA refers to pre-existing ADA at the pre-dose assessment on Day 1.

The Safety population included all enrolled participants who had received at least 1 dose of study drug, regardless of the amount of study drug administered. During the study, participants miss few scheduled site visits for sample collection and only participants with data collected at specific timepoints are reported.

A complete neurological examination included assessment of mental status, motor function and balance, sensory exam, newborn and infant reflexes, muscle stretch reflexes in the older children and evaluation of the cranial nerves. The baseline value was defined as the last available value before the first dose of study drug.

Stage 2 of the study was not yet initiated prior to the early termination of the trial.