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Histone H2AX is a ubiquitous member of the H2A histone family that differs from the other H2A histones by the presence of an evolutionarily conserved C-terminal motif, -KKATQASQEY. The serine residue in this motif becomes rapidly phosphorylated in cells and animals when DNA double-stranded breaks are introduced into their chromatin by various physical and chemical means. In the present communication we show that this phosphorylated form of H2AX, referred to as γ-H2AX, appears during apoptosis concurrently with the initial appearance of high molecular weight DNA fragments. γ-H2AX forms before the appearance of internucleosomal DNA fragments and the externalization of phosphatidylserine to the outer membrane leaflet. γ-H2AX formation is inhibited byN-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone and the inhibitor of caspase-activated DNase, and it is induced when DNase I and …