CAPN9 (calpain 9) encodes a calcium-dependent cysteine protease with tissue-specific expression and diverse cellular functions. The protein functions as part of a unique heterodimeric complex called 'G-calpain' with calpain 8, which is predominantly expressed in gastric mucosa and essential for gastric mucosal defense against injury 1. Beyond its gastrointestinal role, CAPN9 demonstrates broader physiological significance in fibrosis regulation, where TGFβ-induced expression of CAPN9 is required for mesenchymal transition and myofibroblast differentiation 2. Mice lacking functional CAPN9 show protection from bleomycin-induced lung fibrosis, carbon tetrachloride-induced liver fibrosis, and angiotensin II-induced cardiac fibrosis, suggesting therapeutic potential for fibrotic diseases 2. In cancer biology, CAPN9 promotes tumor progression in high-grade serous ovarian cancer, where it is negatively regulated by miR-585-3p and supports cell proliferation and migration 3. Additionally, CAPN9 contributes to epithelial cell lumen formation through activation of apoptotic pathways involving PKC-delta 4. The protein is also part of islet-specific metabolic gene networks regulated by NF-κB signaling, indicating roles in glucose homeostasis 5. Interestingly, downregulation of CAPN9 expression correlates with arterial stiffness in healthy individuals, potentially representing a homeostatic response 6.