ESR1 and cancer: For example: (a) CTCs maintained higher expression than all tumor cell lines for FOXC1, KRT18, PTEN, NPTN, TGFß1, KRT8, ZEB2, and CXCR4; (b) cancer cell lines displayed measurable CDH1 expression, but only 2/105 CTCs expressed transcripts for this epithelial cell adhesion protein that is down-regulated in EMT; and (c) all single cells within cancer cell lines of known molecular subtypes maintained ER, PR, EGFR, and HER2 expression, whereas only 1, 1, 1, and 6/105 CTCs, respectively, displayed these clinically-informative phenotypes.