Peptide bond hydrolysis is one of the final common events leading to cellular injury and death. Excessive proteolytic activity occurs in virtually all disease states including cancer, atherosclerosis, autoimmunity and infection. To counter the deleterious effects of proteolytic activity, cells express several groups of anti-proteases, of which the serine protease inhibitors (serpins) superfamily is the largest. Serpins evolved over 1000 million years and employ a unique suicide substrate-like inhibitory mechanism to neutralize target proteases. We have characterized a novel subset of serpins that, unlike the canonical circulating serpins such as α1-antitrypsin (SERPINA1), reside predominantly within the cell. The intracellular serpins (serpinsIC) are conserved in all metazoa, lack typical signal peptides and neutralize a diversity of serine and, paradoxically, cysteine proteases. We are using transgenic models in mice and the small nematode, C. elegans, to determine how serpinsIC protect both the intracellular and pericellular milieu of multicellular organisms from the collateral damage inflicted by promiscuous proteolysis and protection from cell death.