E2F1 and retinoblastoma: To confirm that E2F1 did play a causal role in promoting transcription of CKS2, we knocked down E2F1 with two short hairpin RNAs (shRNAs) in human retinoblastoma cells (Y79 and WERI-Rb-1) and found that expression of CKS2 was significantly decreased in both RNA and protein levels (Fig. 3B–D).