A synthetic peptide inhibitor that inhibits caspase-4 and related caspase activity. FMK inhibitors are irreversible and non-toxic, and can be used in both in vivo and in vitro studies. The inhibitor is designed as a methyl ester to facilitate cell permeability. (CAUTION: If the intended use is on purified or recombinant enzymes, esterase should be added to generate free carboxyl groups).