MDM2 and viral infectious disease: However, only a few E3 ubiquitin ligase proteins have been reported as regulators of RB: during virus infection, cullin 2 is able to target RB for degradation via the human papillomavirus protein E7; SCFSKP2 is also able to target RB via the Epstein-Barr EBNA3C protein; and under non-viral infection conditions, RB is ubiquitinated by NRBE3 and MDM2 [19–23].