Caspase-3 plays an important role in programmed cell death as an execution-phase caspase in degradation of many substrate proteins. We identified a naturally occurring short caspase-3 variant (caspase-3s) from a human carcinoma cell line that is resulted from alternative mRNA splicing. Analysis of nucleotide sequence reveals a deletion of the exon 6 in this variant that resulted in an altered reading frame in the C-terminus, leading to an altered amino acid sequence and a truncated protein. Caspase-3s shares the same amino acid sequence as caspase-3 in the N-terminus containing the prodomain and the majority of the large subunit. The variant is 95 amino acid residues shorter at the C-terminus and is missing the conserved QACRG sequence in the catalytic site. Caspase-3 and caspase-3s are coexpressed in all human tissues examined. Several cancer cell, lines also show coexpression of caspase-3 and caspase-3s, both at the mRNA and protein levels. Overexpression of caspase-3s in 293 cells is more resistant to apoptosis induced by proteasome inhibition. Furthermore, we identified that proteasome inhibition stabilized the level of caspase-3s.