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 (PubMed:25946230). The secreted form promotes differentiation of T helper 1 cells (Th1) (PubMed:25946230). Interacts (via N-terminal region) with MYRF; the interaction inhibits MYRF self-cleavage. Secreted by exosomes through a non-classical pathway. Expressed in differentiated oligodendrocytes in early postanatal central nervous system tissues (PubMed:30249802). Expressed by CD4(+) T cells, the expression increases upon activation (at protein level) (PubMed:25946230). In the spinal cord region, expression is initially detected in the ventral white matter at 18.5 dpc, up-regulates rapidly afterward, and peaks at early postnatal stages from P4 to P7. At P15, expression starts to be detected in the gray matter of spinal cord but gradually disappears thereafter. Also observed in corpus callosum and the white matter of cerebellum at P15 stages. Belongs to the TMEM98 family.