Analysis of thymocytes lacking HDAC1/2 isolated from neonatal mice. (A) Schematic diagram of the model system used. Cre expression is driven from the proximal promoter of the T-cell specific tyrosine kinase, Lck. Both Hdac1 and Hdac2 are inactivated by deletion of exon 2, which is flanked by LoxP sites (triangles). (B) Schematic diagram illustrating the key steps of intra-thymic T-cell development. Early lymphoid progenitor cells enter the thymus from the bone marrow as double negative (DN) cells, the most immature cells of the thymus and exit to the periphery as either CD4SP (helper) MHC class II–restricted, or CD8SP (cytotoxic) MHC class I–restricted T cells. DN1, DN2, DN3, and DN4 stages of DN cell differentiation are distinguished by the relative expression levels of CD44 and CD25. Likewise, stages of DN to CD4⁺ or CD8⁺ T-cell differentiation are distinguished by the relative expression levels of CD4 and CD8. Bottom panel: Expression of other cell-surface markers, in combination with the expression of CD4/8, used to distinguish compartments of intra-thymic development of T cells of the TCRα/β lineage. (C) Thymocytes of the indicated genotype were isolated from neonatal mice (1-2 weeks old) and used either to make nuclear extract for Western blotting (top panel), or for 2 color FACS analysis (bottom panels). Percentages represent the mean where n > 6 for each genotype. (D-E) Comparative thymocyte cellularity compared with litter-mate controls from the genotypes indicated (***P < .001, paired t test). (F) Analysis of double negative (DN) thymocytes. Comparative thymocyte cellularity (left panel), and 2-color FACS analysis (right panel) for thymocytes of the indicated genotype.