ESR1 and neoplasm: An "intrinsic gene set" of 476 cDNAs was then used to cluster and segregate the tumours into four major subgroups: 1) a "luminal cell-like" group expressing ER; 2) a "basal cell-like" group expressing cytokeratins 5 and 17, integrin 4, and laminin, but lacking ER expression; 3) a HER2-positive subset; and 4) a "normal" epithelial group.