Required for progression through the G1 and S phases of the cell cycle and for S phase entry. Activates transcription of the histone H2A, histone H2B, histone H3 and histone H4 genes in conjunction with MIZF. Also positively regulates the ATM, MIZF and PRKDC promoters. Transcriptional activation may be accomplished at least in part by the recruitment of the NuA4 histone acetyltransferase (HAT) complex to target gene promoters (By similarity). Required for early embryonic development. Interacts with the cylin/CDK complexes CCNE1/CDK2 and CCNA1/CDK2. Interacts with BZW1, CASP8AP2, CREBBP, MIZF and YY1. Interacts with the RUVBL1, RUVBL2 and TRRAP subunits of the NuA4 complex. May also interact with GAPDH, NME1, NME2 and STIP1 (By similarity). Concentrates in Cajal bodies tethered to histone gene clusters. Expression peaks at the G1/S phase boundary. The LisH domain is required for the activation of histone gene transcription. Phosphorylated at Ser-771, Ser-773, Ser-1096, Thr-1264 and Thr-1343 by CCNE1/CDK2 at G1-S transition and until prophase, which promotes association with histone gene clusters and stimulates activation of histone transcription. Also phosphorylated by CCNA1/CDK2 in vitro (By similarity). Belongs to the NPAT family.