Fig. 7. DNA damage sensitivity assays. (A and B) UV sensitivity of MEFs from SIRT1^+/Δ intercross (A), and SIRT1^+/Δex4 intercross (B). XPF^–/– MEFs serve as a positive control for increased UV sensitivity. Passage 3 MEFs (4 ´ 10⁵ cells) were plated onto gelatinized 10-cm dishes, and were irradiated with a 30-watt, short-wave UV-C light source (254 nm) from a distance of 1 m for the indicated times. After 6 days, cells were trypsinized, stained with Trypan blue, and counted. (C and D) IR sensitivity assays of MEFs from SIRT1^+/Δ intercross (C), and SIRT1^+/Δex4 intercross (D). Passage 3 MEFs (4 ´ 10⁵) were treated with 0, 100, 200, 400, or 800 rads of γ-irradiation from a 137Cr source, and were plated onto gelatinized 10-cm dishes. After 6 days, the cells were trypsinized, stained with Trypan blue, and counted.