Rapidly and transiently inhibited by phosphorylation following the generation of DNA double-stranded breaks during S-phase. This is cell cycle checkpoint and ATM-pathway dependent and appears to regulate processes involved in chromatin assembly (By similarity). Isoform 3 protects the cells from the ionizing radiation by facilitating the repair of DSBs. In vitro, phosphorylates histone H3 at 'Ser-10' (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Cell-cycle regulated, maximal activity in S-phase. Inactivated by phosphorylation at Ser-743, potentially by CHEK1 (By similarity). Heterodimerizes with TLK2. Interacts with ASF1A and ASF1B (By similarity). Ubiquitously expressed in all tissues examined. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.