Background of the antigen
E3 ubiquitin-protein ligase that mediates polyubiquitination of CD4. E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfer the ubiquitin to targeted substrates. May play a role in ubuquitin-dependent protein sorting in developmenting spermatids.