Accumulations of phosphorylated H2AX histone proteins (referred to as γH2AX) inside cell nuclei that can be detected with monoclonal antibodies and observed under a fluorescence microscope. γH2AX frequency is a sensitive marker of DNA double strand breaks.
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from ICRP Publication 131, 2015
Identification of the broken ends of DNA caused by ionising radiation. H2AX is one of several genes coding for histone H2A. H2AX becomes phosphorylated on serine 139, then called ‘γH2AX’, as a reaction on DNA double-strand breaks (DSBs). γH2AX is a sensitive target for looking at DSBs in cells.