Histone H2AX undergoes phosphorylation on Ser 139 (gamma-H2AX) rapidly in response to DNA double-strand breaks induced by exogenous stimuli, such as ionizing radiation. However, the endogenous phosphorylation pattern and modifier of H2AX remain unclear. Here we show that H2AX is regulated physically at the level of phosphorylation. at Ser 139 during a hair cycle in the mouse skin. In anagen hair follicles, gamma-H2AX-positive cells were observed in the outer root sheath (ORS) and hair bulb in a cycling inferior region but Dot in a permanent superficial region. In telogen hair follicles, gamma-H2AX-positive cells were only detected around the germ cell cap. In contrast, following X-irradiation, gamma-H2AX was observed in various cell types including the ORS cells in the permanent superficial region. Furthermore, gamma-H2AX-positive cells were detected in the skin of mice lacking either ATM or DNA-PK, suggesting that these kinases are not essential for phosphorylation in vivo. (C) 2007 Elsevier Inc. All rights reserved.