The haspins are divergent members of the eukaryotic protein kinase family that 
are conserved in many eukaryotic lineages including animals, fungi, and plants. 
Recently-solved crystal structures confirm that the kinase domain of human 
haspin has unusual structural features that stabilize a catalytically active 
conformation and create a distinctive substrate binding site. Haspin localizes 
predominantly to chromosomes and phosphorylates histone H3 at threonine-3 during 
mitosis, particularly at inner centromeres. This suggests that haspin directly 
regulates chromosome behavior by modifying histones, although it is likely that 
additional substrates will be identified in the future. Depletion of haspin by 
RNA interference in human cell lines causes premature loss of centromeric 
cohesin from chromosomes in mitosis and failure of metaphase chromosome 
alignment, leading to activation of the spindle assembly checkpoint and mitotic 
arrest. Haspin overexpression stabilizes chromosome arm cohesion. Haspin, 
therefore, appears to be required for protection of cohesion at mitotic 
centromeres. Saccharomyces cerevisiae homologues of haspin, Alk1 and Alk2, are 
also implicated in regulation of mitosis. In mammals, haspin is expressed at 
high levels in the testis, particularly in round spermatids, so it seems likely 
that haspin has an additional role in post-meiotic spermatogenesis. Haspin is 
currently the subject of a number of drug discovery efforts, and the future use 
of haspin inhibitors should provide new insight into the cellular functions of 
these kinases and help determine the utility of, for example, targeting haspin 
for cancer therapy.