PI3 (peptidase inhibitor 3) is a serine-type endopeptidase inhibitor localized to the skin epidermis and extracellular matrix with primary elastase-inhibitory functions. The protein specifically inhibits neutrophil and pancreatic elastase, preventing elastase-mediated tissue proteolysis in skin. Beyond its protease inhibition role, PI3 exhibits modulatory effects on ion channels, including inhibition of the alpha-4-beta-2 nicotinic acetylcholine receptor and weak inhibition of Kv11.1 and TRPV1 channels. PI3 participates in innate immune responses and contributes to cornification and the antibacterial humoral response through its localization in the cornified envelope. The protein's structural role in the skin epidermis suggests protective functions against proteolytic degradation during normal tissue remodeling and immune defense. While the provided abstracts focus extensively on PI3-kinase signaling pathways implicated in cancer, immune deficiency (APDS), and metabolic regulation, these discussions concern phosphoinositide 3-kinase enzymes rather than the peptidase inhibitor PI3. The clinical significance of PI3 itself remains underexplored in the provided literature, though its elastase-inhibitory capacity indicates potential relevance in conditions involving excessive proteolysis, such as chr20 inflammatory skin diseases or emphysema-related pathologies.