Phi c31 integrase is investigated as a novel tool for nonviral gene therapy as the enzyme can direct site-specific integration into a host chromosome. In order to investigate effects of phi c31 integrase expression in normal human cells, we have generated stably transfected primary human fibroblasts expressing the enzyme. All control cells were cytogenetically normal, but in cells expressing phi c31 integrase, numerous chromosomal abnormalities including various translocations were found, suggesting that the enzyme itself acts as a mutagen.