ESR1 and breast cancer: Epi.CTC counts were also found to be elevated in BC patients with estrogen receptor (ER) positive (mean = 14.78, median = 2.18, range = 0–50.10, n = 9) compared to ER negative (mean = 1.93, median = 2.44, range = 0–3.83, p = 0.072, n = 5) tumor status.