PARP1 and cancer: Besides PARP1 itself, S-ADPr has been found as a widespread post-translational modification that targets hundreds of proteins, such as histones and high-mobility group (HMG), those proteins have important roles in chromatin regulation, DNA repair, and cancer progression, which imply the significant of S-ADPr in the regulation of DNA damage response and the maintenance of genome stability [5,6,8].