A yeast expression plasmid encoding a mini-proinsulin molecule was constructed and transformed into Saccharomyces cerevisiae. The plasmid encoded the sequence: B-Arg-Arg-Leu-Gln-Lys-Arg-A in which B represents the B-chain (30 amino acid residues) and A represents the A-chain (21 amino acid residues) of human insulin. The secreted peptides were shown to be a mixture of human insulin and des(B-30)human insulin. Thus, correct disulphide bridges can be established in proinsulin-like molecules devoid of a normal C-peptide region. Furthermore, the specificity of the yeast processing enzymes is so similar to the proinsulin converting enzymes in the human pancreatic beta-cell that it allows the processing of the mini-proinsulin to insulin.