References for this review were identified through searches of PubMed for articles published from January 1970, to October 2022, by use of the terms “prolactin”, “placental lactogens”, “prolactin receptors”, “dopamine”, “dopamine receptor agonist, antagonist”, “insulin”, “insulin resistance”, “hyperprolactinemia”, and “hypoprolactinemia” in combination with the term “diabetes”. The gene discussed is INS; the disease is diabetes mellitus.