Post-translational modifications of conserved N-terminal tail residues in 
histones regulate many aspects of chromosome activity. Thr 3 of histone H3 is 
highly conserved, but the significance of its phosphorylation is unclear, and 
the identity of the corresponding kinase unknown. Immunostaining with 
phospho-specific antibodies in mammalian cells reveals mitotic phosphorylation 
of H3 Thr 3 in prophase and its dephosphorylation during anaphase. Furthermore 
we find that haspin, a member of a distinctive group of protein kinases present 
in diverse eukaryotes, phosphorylates H3 at Thr 3 in vitro. Importantly, 
depletion of haspin by RNA interference reveals that this kinase is required for 
H3 Thr 3 phosphorylation in mitotic cells. In addition to its chromosomal 
association, haspin is found at the centrosomes and spindle during mitosis. 
Haspin RNA interference causes misalignment of metaphase chromosomes, and 
overexpression delays progression through early mitosis. This work reveals a new 
kinase involved in composing the histone code and adds haspin to the select 
group of kinases that integrate regulation of chromosome and spindle function 
during mitosis and meiosis.