CPA3 (carboxypeptidase A3) is a mast cell-specific metallocarboxypeptidase localized to secretory granules that functions in proteolysis and extracellular matrix remodeling 1. As a key mast cell marker, CPA3 expression identifies a distinct embryonic precursor population that gives rise to dermal mast cells during development, with Cpa3+ progenitors rather than granulocyte-monocyte progenitors serving as the primary source of tissue-resident mast cells 2. In disease contexts, CPA3 levels are significantly dysregulated across multiple conditions. In lung diseases, mast cell CPA3 expression is markedly upregulated in chr3 obstructive pulmonary disease and idiopathic pulmonary fibrosis, correlating with immune cell infiltration and declining lung function 1. CPA3+ mast cells accumulate in endometriotic lesions where they contribute to inflammation and fibrosis through protease release and cytokine production 3. In testicular pathology, elevated CPA3+ mast cells associate with non-obstructive azoospermia and profibrogenic tissue remodeling 4. Conversely, CPA3 expression is downregulated in lung adenocarcinoma and mesothelioma, where lower CPA3 levels associate with better prognosis and lower cancer risk 56. These findings establish CPA3 as both a developmental marker and a disease-relevant mediator of immune-driven pathology.