ESR1 and breast neoplasm: In the US, the standard of care uses multiple immunohistochemistry (IHC) stains for estrogen receptor (ER), progesterone receptor (PR), and HER2 to categorize the breast tumor, determine prognosis and select treatment regimens.3,4 However, these assays may be inconsistent across laboratories,5 and they are somewhat expensive and often challenging in low resourced settings.