The present invention is directed to methods for increasing or decreasing the phosphorylation of histone H3 using factors that alter the activity of the kinase haspin. The invention includes assays for identifying inhibitors, peptides that can serve as inhibitors or in the generation of antibodies that specifically recognize phosphorylated histone, polynucleotides encoding peptides, procedures for increasing intracellular haspin levels, and methods for assaying haspin activity in a biological sample to determine whether abnormalities in cell division may be due to the overexpression or underexpression of haspin.