As prolactin and progesterone have synergic roles to induce cell growth and proliferation during adult gland maturation and alveologenesis of the breast terminal duct-lobular units, the site of origin for most breast cancers, a crosstalk between progesterone, prolactin and receptor signaling pathways may not only be relevant in normal, but also malignant breast cells [46]. This evidence concerns the gene PRL and breast cancer.