PCSK1 mRNA (encoding both PC1 and 3 isoforms) was significantly decreased in T1D pancreata, whereas expression of PCSK2 (encoding PC2) and CPE were not affected [115], suggesting that the defect in proinsulin processing arises as a result of diminished PC1/3 activity. This evidence concerns the gene PCSK1 and type 1 diabetes mellitus.