Updated project metadata. Human breast cancer cell lines were cultured in phenol red-free RPMI1640 medium supplemented with 10% fetal bovine serum and 1nM E2. All cell lines were banked in multiple aliquots to reduce the risk of phenotypic drift and identity confirmed using STR. Cells were routinely screened for mycoplasma contamination. Long-term E deprived (LTED) cells modelling resistance to an AI were derived from all parental cell lines as previously described. Two models of MCF7-LTED cells were available: one harbouring a Y537C mutation in ESR1 and the other with wt-ESR1. SUM44-LTED was shown to harbour a natural heterozygous ESR1Y537S mutation. Elacestrant resistant cell lines were generated by growing parental cells (MCF7-LTEDY537C) long-term in the presence of RPMI1640 containing 10% DCC + 0.1nM E2 + 1μM elacestrant. All cell lines were stripped of steroids for 48-72 hours prior to the start of experiments.