CD45RO is one isoform among multiple derived from the same gene, PTPRC, a protein tyrosine phosphatase. CD45RO is located on activated and memory T lymphocytes, and it functions to facilitate T cell activation. In immunohistochemistry, CD45RO is an effective marker of activated T cells and is used in the diagnosis of lymphoma. In normal tissue, it is also positive on granulocytes, monocytes, macrophages, and cortical thymocyes (additionally, while it is mostly negative on B cells, it will show occasional positivity). In disease, in addition to T-cell lymphomas, it is positive in some myelomas, histocytic sarcomas, and AML. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; J Clin Pathol 1998;51:432, PMID: 9771441;