The transient mitotic histone H3 phosphorylation by various protein kinases 
regulates chromosome condensation and segregation, but the counteracting 
phosphatases have been poorly characterized [1-8]. We show here that PP1γ is the 
major histone H3 phosphatase acting on the mitotically phosphorylated (ph) 
residues H3T3ph, H3S10ph, H3T11ph, and H3S28ph. In addition, we identify 
Repo-Man, a chromosome-bound interactor of PP1γ [9], as a selective regulator of 
H3T3ph and H3T11ph dephosphorylation. Repo-Man promotes H3T11ph 
dephosphorylation by an indirect mechanism but directly and specifically targets 
H3T3ph for dephosphorylation by associated PP1γ. The PP1γ/Repo-Man complex 
opposes the protein kinase Haspin-mediated spreading of H3T3ph to the chromosome 
arms until metaphase and catalyzes the net dephosphorylation of H3T3ph at the 
end of mitosis. Consistent with these findings, Repo-Man modulates in a 
PP1-dependent manner the H3T3ph-regulated chromosomal targeting of Aurora kinase 
B and its substrate MCAK. Our study defines a novel mechanism by which PP1 
counteracts Aurora B.