ACP5 (Acid phosphatase 5, Tartrate-resistant acid phosphatase, TRAP) is an iron-containing glycoprotein that catalyzes the conversion of orthophosphoric monoester to alcohol and orthophosphate. Broadly, it functions in skeletal development, bone mineralization, collagen synthesis and degradation, as well as dendritic cell maturation and macrophage recruitment. ACP5 is the most basic of the acid phosphatases and is the only form not inhibited by L(+)-tartrate. It is expressed at high levels by osteoclasts, macrophages and dendritic cells. Two forms circulating in human blood (designated 5a and 5b) are derived from different cell types and have different functions. 5a is derived from macrophages and dendritic cells and is a marker of inflammatory conditions. 5b is derived from osteoclasts and is a marker of bone resorption. It is believed to initiate osteoclast differentiation and proliferation, and is capable of degrading skeletal phosphoproteins such as osteopontin. Separately, ACP5 is thought to function in hyperplastic obesity, and is associated with morbid obesity in mice when overexpressed. In immunohistochemistry, ACP5 has membranous staining. It is a useful positive marker for B lymphocytes of the marginal zone and osteoclasts in normal tissue, and for a range of cancers including hairy cell leukemia, mantle cell lymphoma, primary mediastinal B cell lymphoma, and some splenic marginal zone lymphomas. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; Autoimmunity. 2008. 41(3), DOI: 10.1080/08916930701694667; Biomarkers. 2017. 22 (8), DOI: 10.1080/1354750X.2017.1334155; Hum Pathol 2005;36:945, PMID: 16153456;