The primary objective of the lactation study is to characterize the presence and concentration of BRIUMVI™ in breast milk among breastfeeding participants who receive BRIUMVI™ therapeutically for the treatment of relapsing forms of multiple sclerosis (RMS).

Inclusion Criteria:

Maternal Criteria:

• Participant has independently decided to be treated with BRIUMVI™ prior to providing consent to participate in the study
• Diagnosis of RMS, to include clinically isolated syndrome (CIS), relapsing-remitting multiple sclerosis (RRMS), and active secondary progressive multiple sclerosis (SPMS)
• Willing to breastfeed or pump regularly during the study period to maintain milk supply and exclusively pump breast milk for the 24-hour period of breast milk collection Day 1 post IV dose
• Plans to continue feeding infant breast milk at least throughout the duration of the study and is not weaning

Infant Criteria:

• Gestational age at delivery ≥35 weeks
• Birthweight > 10th percentile
• Weight > 10th percentile as reported by the mother at the time of enrollment

Exclusion Criteria:

Maternal Criteria:

• Any active infection or other condition that would prevent the individual from breastfeeding
• History of breast implants, breast augmentation, or breast reduction surgery that significantly impacts breastfeeding or collection of milk from 1 or both breasts
• History of mastectomy
• Evidence of mastitis or any other significant active infection at Day 1 (pre-dose) that would prevent collection of milk from one or both breasts
• Current use of drugs known to transfer to the breast milk and with established or potential deleterious effects for the infant, including but not limited to aspirin (risk of Reye's syndrome), tetracyclines or fluoroquinolones

Infant Criteria:

- Any abnormality noted or clinically significant medical condition, including cardiac, pulmonary, and liver disease, glucose instability, or active infection at the time of screening that, in the opinion of the investigator, may make implementation of the protocol or interpretation of the trial difficult or would put the infant at risk by participating in the study

The study population is restricted to breastfeeding participants.

The study population will include breastfeeding individuals with RMS who are exposed to BRIUMVI™.