Abstract: Recently, histone H2AX has been proved to play an important role in DNA repair, cell-cycle checkpoints, genomic stability and tumor suppression. The histone variant H2AX is phosphorylated (denoted as γ-H2AX) in response to the induced DNA double-stranded breaks. And γ-H2AX has been shown to have a rapid and sensitive response to DNA double-strand breaks induced by ionizing radiation. Because γ-H2AX is a reliable marker to DNA double-strand breaks, γ-H2AX assay will make great future in detecting DNA double-strand breaks caused by ionizing radiation, especially in low levels of ionizing radiation.