Phospho-Histone H2A.X (Ser139), also known as γH2A.X, is a variant of the histone H2A protein that plays a crucial role in the cellular response to DNA double-strand breaks (DSBs). When DNA damage occurs, such as that induced by ionizing radiation or anticancer drugs, the protein is rapidly phosphorylated at Ser139 by kinases like ATM, ATR, and DNA-PK. This phosphorylation event is one of the earliest responses to DNA damage and is essential for the recruitment of various DNA repair proteins to the site of the break.