We then evaluated the level of expression of ERα-36 in a panel of human breast cancer cell lines, as well as in PDX models.23 We initially confirmed that ERα-36 is expressed both in ERα-positive and in ERα-negative breast tumors,14, 15, 16, 17 (Supplementary Figures S2d and S2e). This evidence concerns the gene ESR1 and breast carcinoma.