Histone chaperone that facilitates histone deposition and histone exchange and removal during nucleosome assembly and disassembly. Cooperates with chromatin assembly factor 1 (CAF-1) to promote replication-dependent chromatin assembly and with HIRA to promote replication-independent chromatin assembly. Promotes homologous recombination-mediated repair of double-strand breaks (DSBs) at stalled or collapsed replication forks: acts by mediating histone replacement at DSBs, leading to recruitment of the MMS22L-TONSL complex and subsequent loading of RAD51. Also involved in the nuclear import of the histone H3-H4 dimer together with importin-4 (IPO4): specifically recognizes and binds newly synthesized histones with the monomethylation of H3 'Lys-9' and acetylation at 'Lys-14' (H3K9me1K14ac) marks, and diacetylation at 'Lys-5' and 'Lys-12' of H4 (H4K5K12ac) marks in the cytosol. Required for the formation of senescence-associated heterochromatin foci (SAHF) and efficient senescence-associated cell cycle exit. Interacts with histone H3 (including both histone H3.1 and H3.3) and histone H4. Interacts with the CHAF1A, CHAF1B and RBBP4 subunits of the CAF-1 complex. Interacts with CABIN1, HAT1, HIRA, NASP, TAF1 and UBN1. Interacts with CDAN1. Found in a cytosolic complex with IPO4 and histones H3 and H4. Interacts with CREBBP. Phosphorylated by TLK1 and TLK2. Highly phosphorylated in S-phase and at lower levels in M-phase. TLK2-mediated phosphorylation at Ser-192 prevents proteasome-dependent degradation. Phosphorylation at Ser-192 by PRKDC in response to DNA damage promotes the histone chaperone activity and ability to replace histones at double-strand breaks (DSBs) at stalled or collapsed replication forks, leading to RAD51 recruitment. Embryonic lethality at mid-gestation (9.5 dpc). Belongs to the ASF1 family.