Atypical E2F transcription factor that participates to various processes such as angiogenesis, polyploidization of specialized cells and DNA damage response. It plays a key role in polyploidization of cells in placenta and liver by regulating the endocycle, probably by repressing genes promoting cytokinesis and antagonizing action of classical E2F proteins (E2F1, E2F2 and/or E2F3).