PSAP (ACPP, prostatic acid phosphatase) is a dephosphorylating enzyme produced in prostatic glandular epithelium. PSAP is a positive biomarker for hyperplastic prostate in immunohistochemistry and may be used to identify prostate origin in metastatic cancers. PSAP distinguishes prostate cancer (positive) from urothelial cancer, mesonephric remnant hyperplasia and clear cell tumors (generally negative). It is also positive in bladder and rectal carcinomas. Higher expression in tumors has been correlated with increasing tumor stage, although it is thought to be a tumor suppressant by means of vitamin D-associated slowing of prostate growth. Lower expression in normal tissue is therefore thought to be a risk factor for the development of cancer. In immunohistochemistry of normal tissue, antibodies to PSAP have cytoplasmic positivity in lysosomal granules in normal prostatic epithelial and ductal cells. References: Virchows Archiv: An International Journal of Pathology 2009-05; 454.5; 573-579, PMID: 19301031; Cancer Research 9/1/1992; 52.17; 4600-4607, PMID:1380886; The Journal of Steroid Biochemistry and Molecular Biology 2005-10; 97.43102; 37-46, PMID: 16076555; Biomolecules & Therapeutics 2013-01; 21.1; 43393, PMID: 24009853; Journal of Immunology (Baltimore, Md.: 1950) 12/15/2001; 167.12; 7150-7156, PMID: 11739538; The American Journal of Surgical Pathology 1991-08; 15.8; 785-790, PMID; 1712549; Appl Immunohistochem Mol Morphol. 2002 Sep;10(3):231-6, PMID: 12373149; Ann Diagn Pathol 2009;13:402, PMID: 19917478;