Background:It has been reported that large increase of leukemia among survivors of atomic bombs or thyroid cancer after the Chernobyl accident. The analysis of these cancers indicated that there was difference in incidence, causal gene or mutation mechanism with a relationship to age at exposure. We reported that there was no difference in incidence and latency in the T-cell lymphomas (TLs) that were induced by irradiation of female B6C3F1 mice weekly with 1.2 Gy X-rays. However, we observed high frequency of LOH on Chromosome 11 in young adult (4 or 8 weeks of age) group. According to the frequency of LOH, the mutation frequency of Ikaros (Ch11) was higher in 4, 8 weeks of age group. In contrast, in infant (1 week of age) group, we observed high frequency of LOH on Chromosome 19. In this report, therefore, we aimed to investigate that age dependency of radiation-induced mouse T-cell lymphomagenesis mechanism(s) by alteration analysis of Pten (Ch19) which was reported mutations in mouse TLs. Materials and Methods: We analyzed the mutation of Pten in mouse TLs, which developed after irradiation to female B6C3F1 mice weekly to 1.2 Gy X-ray for four consecutive weeks starting at infant (1 week) or young adult (4 and 8 weeks) age by sequencing, western blotting and array CGH. Result: We detected point mutations 22%(4/18), 31%(4/13) and 25%(2/8) in 1, 4 and 8 weeks of age group, respectively. In 1 week of age group, we observed no expression of Pten mRNA TLs (17%: 3/18), and array CGH was revealed that genome homo deletion in these TLs. Out of 44%(8/18) in 1 weeks of age group and 31%(4/13) in 4 weeks of age group TLs which observed LOH at Pten locus, 75%(6/8) and 75%(3/4) occur recombination without changing the number of genome, respectively. These Pten alterations and no expression frequency was 61%(10/18) in 1 week of age group higher than 46%(6/13), 38%(3/8) in 4, 8 weeks of age, respectively. Conclusion: We revealed that the recombination at the Pten locus occurred in TLs from 1, 4 weeks of age groups.High frequency of Pten mutations in TLs from 1 weeks of age groups indicated that T-cell lymphomagenesis in infant age depends on Pten more than Ikaros.