Prolactin (PRL) is a peptide hormone secreted from the pituitary gland that allows mammals to produce milk. It is involved in numerous metabolic processes, pancreatic development and immune system regulation. Antibodies to prolactin are often used to sub-classify pituitary adenomas based on degree of prolactin expression. In immunohistochemistry of normal tissue, prolactin has selective positive staining in pituitary tissue. However, extra-pituitary expression of prolactin has also been detected in normal and neoplastic mammary tissue, skin, reproductive tissues, tissues of the immune system and lymphomas. References: J Clin Pathol. 2006 Dec;59(12):1245-53, PMID: 17142570; Pathol Res Pract. 1992 Apr;188(3):296-302, PMID: 1625993; Mol Endocrinol. 2014 May; 28(5): 622–633, PMID: 24694306