Moreover, the use of cardiac glycosides was associated with an increased risk of invasive breast cancer (RR 1.30, 95 % CI 1.14–1.48) among post-menopausal women [73] and the incidence of estrogen receptor-positive breast cancer was significantly higher than that of estrogen receptor-negative breast cancer in women taking cardiac glycosides [74]. Here, ESR1 is linked to breast cancer.