Lyl1 is required for Lmo2-induced differentiation block and thymocyte self-renewal. (A) Absence of DN thymocyte accumulation in Lmo2;Lyl1^−/− mice. Flow cytometric profiles of thymocytes from 2-month-old mice of the indicated genotypes are shown. (B) Cumulative analysis of the percentage of immature (DN) thymocytes in CD2-Lmo2;Lyl1^−/− mice at 2 to 3 months of age. P values were calculated using Student t test. (C) Normalization of thymocyte cellularity in Lmo2;Lyl1^−/− mice. Numbers of various thymocyte subsets in 2-month-old mice of the indicated genotypes (n = 3 per genotype) were determined by flow cytometry. Numbers are given as mean +SD. (D) Lyl1 is required for long-term transplantation of Lmo2-transgenic thymocytes. Thymocytes were isolated from 2-month-old, Lmo2-transgenic mice of the indicated Lyl1 genotypes, and thymocytes derived from one-quarter of a thymus were injected into sublethally irradiated (6.5 Gy) Ly5.1 recipients. Three weeks later, the proportion of donor thymocytes was determined by flow cytometry. P values were determined using Student t test. (E) Loss of Lyl1 restores progenitor importation into the thymus of Lmo2-transgenic mice. Mx;YFP mice of the indicated genotypes were injected with poly(I:C) at 7 weeks of age. Three weeks later, the percentages of YFP⁺ BM progenitors (Lin^–Kit⁺ BM) and thymocytes (THY) were determined.