Abstract: An expression system which provides heterologous proteins expressed by a non-native host organism but which have native-protein-like biological activity and/or structure. Disclosed are vectors, expression hosts and methods for expressing the heterologous proteins. The expression system involves co-expression of protein factor(s) which is/are capable of catalyzing disulphide bond formation and desired heterologous protein(s). The expression system is presented using yeast cells as the preferred host, protein disulphide isomerase (PDI) and thioredoxin (TRX) as the preferred examples of the protein factors and HCV-E2715 envelope glycoprotein and human FIGF as the preferred examples of the heterologous proteins.