ESR1 and cancer: This has permitted a distinction between Estrogen Receptor (ER) expressing luminal cancers (subdivided into luminal A and B according to progesterone Receptor (PR) expression and Ki-67), HER2-positive cancers (overexpression of HER2) and triple negative cancers (no detectable expression of ER, PR or HER2), which are all associated with different prognoses [3].