Cofactor for serine ADP-ribosylation that confers serine specificity on parp1 and parp2 and plays a key role in DNA damage response. Initiates the repair of double-strand DNA breaks: recruited to DNA damage sites by parp1 and parp2 and switches the amino acid specificity of parp1 and parp2 from aspartate or glutamate to serine residues, licensing serine ADP-ribosylation of target proteins. Serine ADP-ribosylation of target proteins, such as histones, promotes decompaction of chromatin and the recruitment of repair factors leading to the reparation of DNA strand breaks. Serine ADP-ribosylation of proteins constitutes the primary form of ADP-ribosylation of proteins in response to DNA damage. Hpf1 acts by completing the active site of parp1 and parp2: forms a composite active site composed of residues from Hpf1 and parp1 or parp2. Hpf1 also promotes tyrosine ADP-ribosylation, probably by conferring tyrosine specificity on parp1. Interacts with PARP1 (via the PARP catalytic domain). Interacts with PARP2 (via the PARP catalytic domain). Interacts with core nucleosomes in a parp1- and parp2-dependent manner. Localizes to DNA damage sites; chromatin localization is dependent on parp1 and parp2. In adult, mainly expressed in gonads. Expressed maternally: detected as early as 0 hour post-fertilization (hpf), before zygotic expression transcripts starts. Morpholino knockdown of the protein causes malformation anbd delayed hatching. Belongs to the HPF1 family.