Ink4a, but not Arf, is up-regulated in old ETPs. (A) Representative immunostaining profiles used to purify ETPs and DN2, DN3, and DN4 thymocytes from young and old mice. The frequency of ETPs in young and old thymi is indicated. (B) Expression of Ink4a relative to gapdh in ETPs from young (4-6 weeks) and old (18-20 months) mice. Values are reported as the mean ratio of expression in old versus young cells. The mean ± SEM based on 3 independent experiments is shown. (C) Changes in expression of Ink4a relative to gapdh in the indicated DN populations. Values are reported as the mean ratio of expression in old versus young cells. The mean ± SEM based on 3 independent experiments is shown. (D) Expression of Bmi-1, Ets-1, Id-1, Ets-2, Ezh2, and Hmga2 relative to gapdh and Let-7b relative to Sno202 in ETPs. Values are reported as the mean ratio of expression in old versus young cells. The mean ± SEM based on 3 independent experiments is shown. The P values are based on comparison of relative levels between old and young cells. ## indicates not detectable.