The mouse histone H2AX (H2AX) has unique C-terminal Ser residues, which are phosphorylated in response to DNA double-strand breaks (DSBs) by ionizing radiation, suggesting that it plays a role in the maintenance of genomic stability. Here, we show that the H2AX protein was detected in most cells in various tissues, and was abundant in the S phase of the cell cycle. Following X-ray irradiation, H2AX was phosphorylated (gamma-H2AX) in the thymus, small intestine and testis. However, H2AX in epithelial cells in the villi of the small intestine were not strongly phosphorylated, even after X-irradiation. Thus, H2AX was expressed in almost all cells. However, the cells that expressed H2AX were not always phosphorylated by X-irradiation, suggesting a different mechanism of kination in those cells.