ESR1 and breast cancer: In accordance with our breast cancer cell line observations, the ER+ breast cancer patient-derived CTCs also exhibited correlation between ERK and GAPDH (rCTCs=0.579, P<0.01; n=20) and between the pairs ERK with β-tubulin (rCTCs=0.691, P<0.01; n=20) and EpCAM with β-tubulin (rCTCs=0.6, P<0.01; n=20).