Overview of the evolutionary history of RB-E2F pathway in metazoan. (A) Summaries of the gene duplication events for E2F, RB and DP proteins in metazoa. The proteins of E2F and RB family from fly and worm were also tried to be classed into suitable subgroup based one sequence and functional data. (B) A simple summary of the evolutionary history of RB-E2F pathway. The protein interaction information of RB-E2F pathway in vertebrates was labeled with black arrows, mainly based on the recent literature . The protein interaction information in invertebrates and some unicellular organisms were predicted by interaction data of their vertebrate orthologues, and were labeled with grey arrows.