STIP1 homology and U bo, Xenopus/Amphibian,-containing protein 1 (STUB1) is an ubiquitin-protein ligase which targets misfolded chaperone substrates towards proteasomal degradation. STUB1 ubiquitinates NOS1 in concert with Hsp70 and Hsp40. STUB1 modulates the activity of several chaperone comple, Xenopus/Amphibian,es, including Hsp70, Hsc70 and Hsp90. STUB1 mediates transfer of non-canonical short ubiquitin chains to HSPA8 that have no effect on HSPA8 degradation. STUB1 mediates polyubiquitination of CYP3A4.