ESR1 and breast cancer: BMI1 was originally identified as an oncogene that cooperates with MYC to induce lymphomas in mice [13], and MYC is commonly amplified in breast cancer, so it is reasonable to use MYC in a transformation protocol that includes BMI1. We show here that lentiviral transduction of HMECs with ERα, BMI1, MYC and TERT leads to the formation of ERα-positive tumours whose growth is dependent on oestrogen.