Abstract. The genome of haploid Saccharomyces
cerevisiaecontains two nonallelic sets of histone H3
and H4 genes. Strains with deletions of each of these
loci were constructed by gene replacement techniques.
Mutants containing deletions of either gene set were
viable, however meiotic segregants lacking both histone
H3 and H4 gene loci were inviable. In haploid
cells no phenotypic expression of the histone gene deletions
was observed; deletion mutants had wild-type
growth 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.