ESR1 and neoplasm: ERα status was assessed in 33 studies; with a total number of 4200 patients analyzed, the overall rate of ERα status conversion in primary vs. recurrent tumor specimens was 20% (95% CI: 16–35%); in particular, HR conversion from ERα-positive (ERα+) to ERα-negative (ERα−) status occurred in approximately 24% of patients (95% CI: 9–20%), while ERα− to ERα+ conversion occurred in 14% of patients (95% CI: 9–20%).