Phospho-gamma H2A.X (Ser139), also known as 纬-H2AX, is a histone variant that plays a crucial role in the DNA damage response (DDR). It is phosphorylated at serine 139 by kinases such as ATM and ATR in response to DNA double-strand breaks (DSBs), which are often caused by ionizing radiation or cytotoxic agents. This phosphorylation event is one of the earliest cellular responses to DNA damage and results in the formation of 纬-H2AX foci in the cell nucleus, marking the sites of DSBs on a one-to-one basis. These foci serve as recruitment signals for DNA repair proteins, facilitating the repair process and maintaining genomic stability. Due to its sensitivity and specificity, 纬-H2AX is widely used as a biomarker for DNA damage and repair in research, including studies on cancer, aging, and environmental genotoxicity.