RB1 and neoplasm: Pull-down using an E2F1 antibody showed enrichment of E2F1 at a consensus-binding site within the Hells promoter in samples that highly express HELLS protein: P0 wild-type and P21 Rb1/p107 DKO retinae and human tumor cells, but not in P21 Cre-negative Rb1/p107 (Fig. 3c).