Restriction factor required to restrict infectivity of gammaretroviruses: acts by inhibiting early step of viral infection and impairing the ability of the viral particle to translocate its content to the cytoplasm (By similarity). Enhances the incorporation of serine into phosphatidylserine and sphingolipids. May play a role in providing serine molecules for the formation of myelin glycosphingolipids in oligodendrocytes (By similarity). Localizes to the cell membrane, where it is efficiently incorporated into budding gammaretrovirus virions and impairs subsequent virion penetration of susceptible target cells. Belongs to the TDE1 family.