The E2F and DP protein families form heterodimeric transcription factors that play a central role in the expression of cell cycle-regulated genes. The crystal structure of an E2F4-DP2-DNA complex shows that the DNA-binding domains of the E2F and DP proteins both have a fold related to the winged-helix DNA-binding motif. Recognition of the central c/gGCGCg/c sequence of the consensus DNA-binding site is symmetric, and amino acids that contact these bases are conserved among all known E2F and DP proteins. The asymmetry in the extended binding site TTTc/gGCGCc/g is associated with an amino-terminal extension of E2F4, in which an arginine binds in the minor groove near the TTT stretch. This arginine is invariant among E2Fs but not present in DPs. E2F4 and DP2 interact through an extensive protein-protein interface, and structural features of this interface suggest it contributes to the preference for heterodimers over homodimers in DNA binding.