Thymic T-cell development in Rag1-mutant mice. (A) Tissue sections of thymus stained with hematoxylin and eosin for medulla (lighter staining) and cortex (darker staining). Original magnification ×4. (B) Live cell counts from individual thymuses of WT, hypomorphic Rag1-mutant, and Rag1 knockout (KO) mice. Thymocyte developmental stages were analyzed by flow cytometry for double-negative (DN; CD4⁻CD8⁻) cells, double-positive (DP; CD4⁺CD8⁺) cells, and single-positive (CD4⁺ or CD8⁺) cells (C); lineage-negative DN populations, DN1 (CD44⁺CD25⁻), DN2 (CD44⁺CD25⁺), DN3 (CD44⁻CD25⁺), and DN4 (CD44⁻CD25⁻) (D), and thymocytes expressing the αβ or γδ form of the TCR (E). Representative flow cytometry panels with 6 thymuses per group (open circles). Error bars represent standard error of the mean. Statistical analysis was performed with 1-way analysis of variance. *P ≤ .05, **P ≤ .01, ***P ≤ .001, ****P ≤ .0001.