Indeed, while Rasmussen et al. demonstrated that PARP7functions as a tumor suppressor in E2-responsive breast cancer cells,154 a PARP7 inhibitor reached clinical trials forits promising antitumor activity.155 Inthe first study, the authors provided evidence that PARP7 is a fundamentalmember of the negative feedback that regulates estrogen receptor α(ERα). This evidence concerns the gene ESR1 and neoplasm.