ESR1 and breast cancer: In order to address the potential mechanism underlying the lack of clinical benefit gained from abiraterone in ER+ metastatic breast cancer patients, we investigated its impact on ER signalling in a panel of ER+ breast cancer cell lines sensitive (i.e., dependent on oestrogen for their proliferation) or resistant to long-term oestrogen-deprivation (LTED), modelling relapse on an aromatase inhibitor (AI), and harbouring various naturally occurring ESR1 mutations.