The genetic and transcriptomic alterations associated with expression of ER mutants have been widely investigated in breast cancer cell lines in which ER mutants arise spontaneously14, following ectopic expression5,6,8 or by CRISPR-Cas9-directed knock-in mutagenesis of the endogenous ESR1 gene9,10,15,16. Here, ESR1 is linked to breast carcinoma.