An accumulation of DNA damage in long-lived human HSCs. (A) Lin⁻CD34⁺CD38⁻ cells or Lin⁻CD34⁺CD38⁺ cells isolated from CB and recipient mice bone marrow at 18 weeks after primary (n = 10) and secondary (n = 15) transplantation were immunostained for γ-H2AX (γ-H2AX: green; DAPI: blue; left). All bars represent 5 μm. The box and whisker plot shows the number of γ-H2AX foci per cell. More than 50 cells in random fields on a slide were counted for 5 independent experiments. (B) Lin⁻CD34⁺CD38⁻ cells were immunostained with antibodies for ATM phosphorylated on Ser 1981 (green), 53BP1 phosphorylated on Ser 1778 (green), CHK2 phosphorylated on Thr 68 (green), or FOXO3a (green). DAPI (blue) was used to stain nucleus. All bars represent 5 μm.