Updated publication reference for PubMed record(s): 28190768. We report that serine ADPr is strictly dependent on HPF1 (histone PARylation factor 1. Quantitative proteomics revealed that serine ADPr does not occur in cells lacking HPF1. We identified three endogenous serine ADPr sites are located on the PARP-1 automodification domain. Further identification of serine ADPr on hundreds of other targets indicates that serine ADPr is a widespread modification.