Reversible protein phosphorylation by kinases and phosphatases is one of the most common mechanisms in controlling most,if not all cellular processes.The phosphorylated form at serine-139 of the histone H2A variant H2AX,γ-H2AX,is a marker for DNA double-stranded breaks (DSBs),and the elimination of γ-H2AX at DNA damage foci is required for DNA damage repair.