CUL2 and retinoblastoma: Although E6-mediated elimination of PDZ domain proteins requires the human UBE3A, the ubiquitination and subsequent proteasomal degradation of Drosophila Retinoblastoma by E7 did not require the human cullin 2 ubiquitin ligase complex [94], suggesting that the Drosophila counterpart represents a conserved binding activity of E7.