Saccharomyces cerevisiae contains at least four PRS genes, all of which have been cloned and sequenced. Each of the four derived amino acid sequences have more than 60% similarity to the corresponding polypeptides of man, rat, Escherichia coli and Salmonella typhimurium. The PRS1 gene maps on chromosome XI, PRS2 on chromosome V, PRS3 on chromosome VIII and PRS4 on chromosome II. One member of this gene family, PRS1, contains a region of non-homology (NHR) shown by cDNA cloning and sequencing not to be an intron. The results presented here suggest that the presence of this NHR is not detrimental to the function of the gene. To date the possibility of protein splicing can be neither proven nor disputed.