ESR1 and breast neoplasm: In 2000, Perou et al. proposed the first “molecular portrait of human breast tumors” based on complementary DNA (cDNA) microarrays that classified breast tumors into four molecular types [303]: luminal epithelial/estrogen receptor positive (ER+), ERBB2-overexpressing/HER2-enriched, basal epithelial-like, and normal breast-like.