This study looks at how much BRIUMVI™ (Ublituximab) is found in breast milk. BRIUMVI™ is a medicine used to treat relapsing forms of multiple sclerosis (RMS), a disease that affects the brain and spinal cord. The study is for breastfeeding moms who are already using BRIUMVI™. Moms need to be willing to pump breast milk during the study; especially on Day 1 right after getting their IV dose. Both mom and baby should meet certain health criteria to join the study. Moms should not have any active infections or a history of breast surgeries that could affect breastfeeding. Babies should be born at 35 weeks or later and be at a healthy weight. Risks include potential side effects from the medicine to the baby, so moms using certain drugs that pass into breast milk can't participate.

• The study involves collecting breast milk on Day 1 after IV dose.
• Moms must already be using BRIUMVI™ and willing to pump milk during the study.
• Both mother and baby need to meet health criteria to join.