PTPRCAP (protein tyrosine phosphatase receptor type C-associated protein) is a transmembrane phosphoprotein that functions as a regulatory associate of the leukocyte tyrosine phosphatase CD45 1. Located on human chromosome 11, PTPRCAP consists of two exons separated by a 1.2-kb intron 1. As a positive regulator of CD45, PTPRCAP modulates Src family kinase activation in immune cells 2. The protein is expressed in multiple immune cell types including NK cells, macrophages, monocytes, and plasmacytoid dendritic cells, where it contributes to interferon-mediated inflammatory responses 34. Regulatory variants in the PTPRCAP promoter region influence its expression and disease susceptibility. A polymorphism at position -309 (rs869736) increases PTPRCAP expression and correlates with 2.5-fold elevated susceptibility to diffuse-type gastric cancer through enhanced Src family kinase activation 2. Conversely, PTPRCAP functions as a tumor suppressor in lung adenocarcinoma; downregulation occurs frequently in tumor tissue compared to adjacent normal tissue and correlates with advanced TNM stage 5. PTPRCAP overexpression suppresses lung cancer cell proliferation, migration, and invasion while promoting apoptosis via Bax/caspase-3 pathway activation 5. PTPRCAP is also upregulated in immune-mediated inflammatory conditions including vulvar and oral lichen planus, suggesting context-dependent roles in immune regulation 6.