A human-like proinsulin gene and its analogs, have been synthesized by a combination of chemical and enzymatic methods. A number of different human-like proinsulin gene analogs with altered C-chains have also been designed and can be readily constructed as described. As a part of the strategy, an adaptor for trimming DNA has been designed, synthesized and used to recover the A-chain insulin gene with the desired sequence from a hybrid plasmid; a related adaptor for trimming DNA has been designed to shorten the C-chain gene or any gene. The synthetic proinsulin gene has been joined to a replicable cloning vehicle and the hybrid DNA transferred to a host cell. The transformed host cell has been shown to contain the desired human-like proinsulin gene.