BIIB129 is a covalent, selective, small molecule inhibitor of Bruton's tyrosine kinase (BTK) capable of penetrating the blood-brain barrier. BIIB129 inhibits the activity of BTK by covalently binding to Cys481 in BTK, thereby affecting the function of B cells and myeloid cells. BIIB129 can be used in multiple sclerosis (MS) research[1].