3,4-Dichloroisocoumarin is a potent serine-protease and SrLip inhibitor (Ki for SrLip: 26.6 μM). 3,4-Dichloroisocoumarin is opened by serine proteases and then undergoes acylation with the enzyme, thereby inhibiting protease activity. 3,4-Dichloroisocoumarin can induce DNA fragmentation