We report the first discovery of naturally occurring ESR1Y537C and ESR1Y537S mutations in MCF7 and SUM44 ESR1-positive cell-lines after acquisition of resistance to long-term-estrogen-deprivation (LTED) and subsequent resistance to fulvestrant (ICIR). Overall design: RNAseq examination of gene expression changes due to long term estrogen deprivation in two breast cancer cell lines with and without ESR1 mutations