An alternative splice variant of human thioredoxin (hTRX) has been identified by using reverse transcription-polymerase chain reaction, cloning and expressing of hTRX-encoding gene, and DNA sequencing. This variant contains hTRX exons 1, 2, 4 and 5. The entire protein encoding region of this variant, named HTRXδ3, is identical to hTRX except for the omission of exon 3. Human TRX and TRXδ3 cDNA has been expressed in E. coli. The catalytic activity of rhTRXδ3 on DTT-dependent insulin reduction is lower than that of rhTRX, although hTRXδ3 still contains the active site.