A 355 base pair DNA sequence coding for human preproinsulin has been assembled by joining 55 synthetic deoxyoligonucleotide fragments prepared by the modified phosphotriester methodology. Proinsulin was expressed underlac promoter control and truncatedΒ-galactosidase 590 amino acid long sequence. The fusedΒ-galactosidase proinsulin protein was produced in amount to 30 % of the totalEscherichia coli proteins. It was also expressed in M13 bacteriophage and yeast system.