FRT significantly improved the histopathological change of thymus and reduced the rate of apoptosis after radiation by PARP-1. (A) WT and KO mice were randomly divided into 4 groups according to their body weight ① Normal group, ② Radiation group, ③ Radiation +FRT 30 mg/kg, ④ Radiation +FRT 60 mg/kg). Mice were given different doses of FRT by gastrointestinal administration for 4 days, and were irradiated at the dose of 6 Gy. Thymus was removed 4 days after irradiation. The tissues were fixed overnight with 4% paraformaldehyde, and then embedded in paraffin. After sectioning, the tissues were stained with hematoxylin and eosin (n=5). (B) Thymus cells were pretreated with or without FRT (50 and 100 μg/mL) prior to 6 Gy irradiation. Cells were harvested, and apoptosis was assessed by FCM (staining with both Annexin V-FITC and PI) 6 h after irradiation. The percentages of survival cells were compared (** P<0.01 compared with normal group; ## p < 0.01 compared with radiation group; ΔΔ P<0.01 compared with the KO mouse radiation group). PARP-1 knockout in mice can reduce apoptosis rate of thymus cells after radiation injury. Data was expressed as mean ±SD, n=5.