E2F1 and cancer: Moreover, an unbiased location analysis of E2F1‐binding sites in HeLa cells demonstrated that a large fraction (25–35%) of genes in human cancer cells have E2F1 binding on their promoters and the vast majority of these actual E2F1‐binding sites in cells lack the canonical E2F1‐binding motifs (Bieda, Xu, Singer, Green, & Farnham, 2006).