Our goal was, therefore, to study the associations between circulating serum concentrations of E2, P4 and PRL, and the expression of genes encoding for their respective receptors, i.e., ERα (ESR1), ERβ (ESR2), PGR, PRLR, PRL, as well as GHR, in normal mammary tissues as well as in mammary neoplasms in female dogs, taking into account their reproductive state and cycle stage. The gene discussed is ESR1; the disease is breast cancer.