This measurement validation study will use qualitative and quantitative methodology to evaluate the Observer-Reported Communication Ability Measure (ORCA), to appropriately capture communication abilities in individuals with Rett syndrome. The ORCA Measure is a caregiver-reported questionnaire that collects caregiver observations of their child's communication abilities including expressive, receptive and pragmatic communication types. Caregivers will participate via phone interviews and online surveys. Approximately 270 participants will be enrolled.

This study will develop the Observer-Reported Communication Ability Measure (ORCA), originally validated for Angelman syndrome, to appropriately capture communication abilities in individuals with Rett syndrome. The ORCA Measure is a caregiver-reported questionnaire that collects caregiver observations of their child's communication abilities including expressive, receptive and pragmatic communication. The goal of the study is to generate a validated tool to comprehensively assess communication in Rett syndrome and create an important endpoint for use in clinical trials.

In phase 1, a hybrid approach to concept elicitation and cognitive testing will occur with caregivers to assess the existing ORCA measure and evaluate ORCA content validity. Approximately 20 parents/caregivers of individuals with Rett syndrome will participate in qualitative in-depth interviews. The ORCA measure, instructions, recall period, and response choices will be evaluated.

In phase 2, psychometric properties including reliability, floor/ceiling effects, construct validity, and test-retest will be evaluated with approximately 250 caregiver-parents of individuals with Rett syndrome. Caregivers will also complete additional questionnaires to allow comparison between the ORCA measure and existing measures. Analysts will use a variety of statistical methods to evaluate the psychometric properties of the ORCA measure in Rett syndrome.

Inclusion Criteria:

• Parent or caregiver at least 18 years of age, with a female child, age 2 or older, diagnosed with Rett syndrome confirmed by a disease-causing MECP2 genetic mutation.
• Fluency in the English language
• Currently lives with the child with Rett syndrome
• Ability and willingness to participate in all study activities.
• Access to a telephone and/or access to a device with internet access that is capable of supporting electronic survey completion

Exclusion Criteria:

• Parent or caregiver with clinically significant condition or situation that makes them not be suitable for the study, in the opinion of research staff.
• Parent or caregiver of a child with Rett syndrome who also has another diagnosis in addition to Rett syndrome.

Parents/caregivers of individuals with Rett syndrome confirmed by an MECP2 genetic mutation, who are fluent in English.