UBR4 and cervical cancer: For example, the E3 ubiquitin ligase UBR4 binds to the N terminus of E7 and is required for the anchorage-independent growth of HPV-positive cervical cancer cell lines and for the ability of BPV1 E7 to inhibit anoikis (27, 42, 44), but in the absence of a target for ubiquitylation by UBR4, the mechanistic basis of these observations was unclear.