Yeast having modified chromosomes are provided. The chromosomes are modified such that at least one of yeast histones H3, H4, H2A or H2B are fully or partially replaced by their human histone counterparts H3, H4, H2A or H2B, respectively. Histone amino acid substitutions are included. Cell fusions with the yeast having the modified chromosomes and non-yeast cells are provided. Methods for screening test agents using the yeast are also provided. Yeast with a mutated yeast DAD1 gene, the mutated DAD1 gene encoding an E50D mutation in yeast DAD1 protein, are provided, and provide a useful genetic background for making the yeast with partially or fully replaced histone(s).