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The mammalian transcription factor E2F plays an important role in regulating the expression of genes that are required for passage through the cell cycle. This transcriptional activity is inhibited by association with the retinoblastoma tumor suppressor protein (pRB) or its relatives p107 and p103. The first cDNA from the E2F family to be cloned was designated E2F-1, and multiple E2F family members have now been identified. They bind to DNA as heterodimers, interacting with proteins known as DP. Here we demonstrate that DP is also a family of polypeptides with at least two members (hDP-1 and hDP-2). Both hDP-1 and hDP-2 bind to all E2F family members in vivo, and each complex is capable of activating transcription. However, the various E2F/DP complexes display strong differences in the ability to bind to either pRB or p107 in vivo, and the specificity of pRB or p107 binding is mediated by the E2F subunit.