PARP1 and ovarian cancer: In this study, we report for the first time that (i) ETS1 motif methylation is a key regulatory element for PARP1 transcription, (ii) hypomethylated ETS1-mediated increase of histone modification H3K9ac and transcription factor ETS1 enrichment synergistically activate PARP1 expression in BRCA1-mutated ovarian cancer, and (iii) the deletion of H3K9ac and/or ETS1 enrichment were effective ways to induce a decrease of PARP1 levels in ovarian cancer cells.