This is an image of a cell nucleus. The DNA is labeled in blue. Phosphorylated H2AX (a histone variant that is phosphorylated by ATM and ATR at sites of DNA damage) is labelled in green. Mre11 (a double-strand break repair protein) is labelled in red. The merged image shows co-localization of H2AX and Mre11 at sites of double strand breaks.