The protein disulfide isomerase from A. niger was cloned as a series of overlapping DNA-fragments generated using polymerase chain reaction technology and primers derived from conserved regions of published PDI amino acid sequences. The 5’ end of the gene was amplified using inverse PCR. Comparison of amino acid sequences from rat, wheat, yeast and another fungal species shows that the thioredoxin like active sites are strongly conserved. The C-terminus of the fungal PDI contains an endoplasmatic reticulum (ER) retention signal (HDEL) that is preferred by yeast.