Abstract. The genome of haploid Saccharomycescerevisiaecontains two nonallelic sets of histone H3and H4 genes. Strains with deletions of each of theseloci were constructed by gene replacement techniques.Mutants containing deletions of either gene set wereviable, however meiotic segregants lacking both histoneH3 and H4 gene loci were inviable. In haploidcells no phenotypic expression of the histone gene deletionswas observed; deletion mutants had wild-typegrowth rams, were not temperature sensitive for growth, and mated normally. However, diploids
homozygous for the H3-H4 gene deletions were slightly defective in their growth and cell cycle
progression. The generation times of the diploid mutants were longer than wild-type cells, the size distributions of cells from exponentially growing cultures were skewed towards larger cell volumes, and the G1 period of the mutant cells was longer than that of the wild-type diploid. The homozygous deletion of the copy-II set of H3-H4 genes in diploids also increased the frequency of mitotic chromosome loss as measured using a circular plasmid minichromosome assay.