Molecular genetics. Histone-H2AX plays an important role in stimuli induced double-strand break. gamma-H2AX (phosphorylated histone-H2AX) foci formation is a sensitive indicator to detect DNA damage, especially DNA-double strand breaks. Histone-H2AX is an initiation protein which is phosphorylated during the double-strand break repair mechanism. Therefore staining with specific antibodies shows the foci marking double-strand breaks. By repairing double-strand breaks the visible foci decrease.