The gene coding for profilin (PFY), an actin-binding protein, occurs as a single copy in the haploid genome of Saccharomyces cerevisiae and is required for spore germination and cell viability. Displacement of one gene copy in a diploid cell by a nonfunctional allele is recessively lethal: tetrad analysis yields only two viable spores per ascus. The PFY gene maps on chromosome XV and is linked to the ADE2 marker. The primary transcript of about 1,000 bases contains an intron of 209 bases and is spliced into a messenger of about 750 bases. The intron was identified by comparison with a cDNA clone, which also revealed the 3' end of the transcript. The 5' end of the mRNA was mapped by primer elongation. The gene is transcribed constitutively and has a coding capacity for a protein of 126 amino acids. The deduced molecular weight of