GROWTH HORMONE (GH), prolactin (Prl), and chorionic somatomammotropin (CS, placental lactogen) are a family of polypeptide hormones that are related by function, immunochemistry, and structure (1–5). Despite their relatedness, these hormones are synthesized in a tissue-specific manner. GH and Prl are produced in the anterior pituitary, whereas CS is produced by the syncytiotrophoblast of the placenta. All three hormones possess lactogenic and growth-promoting activities. GH is required for postnatal growth and maintenance of nitrogen, mineral, lipid, and carbohydrate metabolism (6), while mammalian Prl is primarily involved in the initiation and maintenance of lactation (7). CS may provide GH-like activities to the developing fetus (8, 9), although it does not appear to be an essential hormone (10).

The three hormones have a similar size (190 to 199 amino acids among various species) and they all have similar protein structures. Each hormone has a single homologous tryptophan residue at about locus 85 (GH and CS) or 91 (Prl), and two homologous disulfide bonds.