ESR1 and neoplasm: Similarly, the ERα status of DTCs could be completely different to that of the primary tumour which on the one hand (ERα-negative primary tumour, ERα-positive DTCs) could increase the number of patients eligible for endocrine therapy and on the other hand (ERα-positive primary tumour, ERα-negative DTCs) could explain why endocrine therapy fails in a subset of hormone receptor-positive patients.