ESR1 and breast cancer: As shown in Fig. 1a, absence of PARP12 specifically induced apoptosis in a subset of breast cancer cells, harbouring the ER expression (i.e. MCF7 and ZR-75–1) whilst not affecting the non-tumorigenic epithelial cell line MCF10A; in addition, no effect was observed in the other cell lines tested (triple negative breast cancer MDA-MB-231 and MDA-MB-468 cells, prostate cancer PC3 cells, colon cancer HCT116 cells and hepatocarcinoma HepG2 cells), suggesting a specificity of PARP12 in cell survival of ER + -breast tumour models.