ESR1 and neoplasm: Indeed, a recent study showed 39.1% of patients treated with AI therapy harboured an ESR1 mutation in circulating tumour DNA and 49% of these were polyclonal.46,47 In order to explore the relevance of ESR1 mutations in response to abiraterone, we used a panel of LTED cell lines that harboured naturally occurring ESR1wt, ESR1Y537C or ESR1Y537S.