Endonuclease, poly(U) specific is a protein that in humans is encoded by the ENDOU gene. This gene encodes a protein with endoribonuclease activity that binds polyuridine-enriched single-stranded RNA. This gene was initially characterized based on its high expression in placenta but was mischaracterized as a serine protease. In mouse, this gene promotes tolerance to self-antigens by regulating B cell activation-induced cell death (AICD). The protein may be useful as a tumor marker. Multiple alternatively spliced transcript variants encoding distinct protein isoforms have been found for this gene.