Functions as a negative regulator of MYRF in oligodendrocyte differentiation and myelination. Interacts with the C-terminal of MYRF inhibiting MYRF self-cleavage and N-fragment nuclear translocation. The secreted form promotes differentiation of T helper 1 cells (Th1). Interacts (via N-terminal region) with MYRF; the interaction inhibits MYRF self-cleavage. Secreted by exosomes through a non-classical pathway. Belongs to the TMEM98 family.