Abstract
A mutant allele of the chromosomal locus corresponding to the cloned actin gene of the yeast Saccharomyces cerevisiae has been constructed by DNA transformation with a hybrid plasmid which integrates into, and thereby disrupts, the protein-encoding sequences of the gene. In a diploid strain of yeast, disruption of the actin gene on one chromosome results in a mutation that segregates as a recessive lethal tightly linked to a selectable genetic marker on the integrated plasmid. The actin gene, therefore, must encode an essential function for yeast cell growth.