ESR1 and neoplasm: In contrast, EGFR/HER1 expression levels correlated negatively with ESR1 tumour levels (r=-0.629, P=0.012, Figure 5C) in addition to intratumour (r=-0.633, P=0.001, Table 5), normal tissue (r=-0.556, P=0.005) and plasma (r=-0.625, P=0.002) E2 levels.