HPF1 (histone PARylation factor 1) is a critical cofactor that regulates PARP1 and PARP2 activity during DNA damage response by switching their amino acid specificity from acidic residues to serine 1. HPF1 forms a composite active site with PARP1/2, providing the catalytic base Glu284 that enables serine ADP-ribosylation of target proteins including histones 2. This serine ADP-ribosylation promotes chr4 decompaction and recruitment of repair factors essential for DNA damage repair 3. Notably, HPF1 exhibits dual regulatory functions: it stimulates PARP1/2 autoPARylation and histone heteroPARylation within specific concentration ranges, while simultaneously restricting polymerase activity by promoting hydrolysis, resulting in shorter poly-ADP-ribose chains and increased free ADP-ribose production 42. Interestingly, HPF1 is largely dispensable for single-strand break repair, as HPF1-deficient cells maintain robust poly-ADP-ribosylation and normal repair capacity 5. HPF1 also facilitates tyrosine ADP-ribosylation, expanding PARP substrate specificity beyond serine residues 6. The protein operates through proteome-wide regulation of serine ADP-ribosylation, primarily as mono-ADP-ribosylation, with consistent targeting motifs independent of HPF1 itself 1.