Interestingly, as a group, patients with metastatic estrogen receptor positive BC, yielded average harvested cell counts from blood that were significantly higher than that of non-metastatic patients, (8,100, range 3,500–25,000 versus 5,270 respectively, p = .005) (Figs 3 and 4), although there was no difference in the average number of input cells (29 million, range 17–49 million), and no obvious differences in collection methods, or other pre-analytical variables. This evidence concerns the gene ESR1 and breast cancer.