PGA5 (pepsinogen A5) is a gastric aspartic-type endopeptidase that serves as an inactive precursor to pepsin, the major acid protease of the stomach 1. The protein exhibits broad substrate specificity, preferentially cleaving bonds involving phenylalanine and leucine residues. PGA5 is part of a polymorphic gene family including PGA3 and PGA4, with genetic variation arising through gene duplication 1. The protein is normally expressed in gastric fundic glands and can be identified immunochemically as an alpha-isozymogen isoform 2. Notably, PGA5 expression extends beyond normal gastric tissue. In Barrett's esophagus, ectopic PGA5 expression induces cancer-associated transcriptomic changes, activating pathways involved in epithelial-mesenchymal transition and tumor microenvironment remodeling through mechanisms involving TGFB1 and ERBB2 signaling 3. Pan-cancer analysis demonstrates PGA5 is expressed in most normal tissues but downregulated in most cancer tissues, correlating with cancer-associated signaling pathways 4. In hepatocellular carcinoma, PGA5-expressing hepatocytes exhibit cirrhosis-like gene expression programs useful for developing diagnostic and prognostic biomarkers 5. These findings suggest PGA5 functions beyond digestion, with clinical significance in cancer diagnosis and prognosis.