The H2A.X Phosphorylation Assay Kit (Flow cytometry) is a cell based assay formatted for flow cytometric detection of levels of phosphorylated Histone H2A.X. Cells are cultured in microplates, treated with agents that induce DNA damage or apoptosis, which stimulates H2A.X phosphorylation. Cells are then fixed and permeabilized in preparation for staining and detection. Histone H2A.X phosphorylated at serine 139 is detected by the addition of the anti-phospho-Histone H2A.X, FITC conjugate. Cells are then scanned in a flow cytometer to quantitate the number of cells staining positive for phosphorylated Histone H2A.X.