We have chosen the Isopenicillin N synthetase (IPNS) gene from Cephalosporium acremonium to study its expression in Escherichia coli due to its peculiar DNA sequence [1]. Significant levels of the protein could not be detected when the IPNS gene was placed under the control of strong promoters. Only when the construction was cloned in a runaway plasmid, significant levels of the protein were found in lysates of E. coli. Besides, the presence of the IPNS gene inhibits expression of a distal gene (galK) in a polycistronic RNA, suggesting that the gene has a low transcriptional efficiency in E. coli.