Based on previous evidence showing a role for PARP-1 in modulating both ERα transcriptional activity and ET in ERα-positive BC cells [20, 61], we evaluated the expression of ERα target genes upon E2 exposure either in the presence of fulvestrant or the PARP-1 inhibitor niraparib in BC cells expressing ERα wild type or Y537S mutation. The gene discussed is ESR1; the disease is breast cancer.