In the present study, we showed that H6PD promotes cancer cell proliferation by using 3 different breast cancer cell models, each representing one of the main molecular subtypes of breast cancer: the triple-negative cell line SUM159 [does not express progesterone receptor (PR−), estrogen receptor (ER−), nor the Her2 receptor (Her2−)], the PR+, ER+, Her2− cell line MCF7, and the PR−, ER−, Her2+ cell line MDA-MB-453. The gene discussed is ESR1; the disease is breast carcinoma.