ESR1 and breast cancer: In two landmark studies, Perou et al. and Sorlie et al. [3,5] identified five distinct "intrinsic" subtypes of breast cancer by hierarchical cluster analysis of microarray gene expression data: luminal A and luminal B [both estrogen receptor-positive (ER+)], HER2 overexpressing (HER2+), normal breast-like and basal-like.