Conditional SCL activation leads to increased NOTCH1 expression. (A) Flow cytometric analysis of CD4⁺CD8⁺ (double-positive, DP) TCRβ^low thymocytes for the expression of NOTCH1 with an antibody (clone mN1A) specific for the intracellular domain of NOTCH1. In addition, analysis with the corresponding IgG₁κ isotype control antibody is also shown (right). Mice received tamoxifen (TAM) feed for 6 weeks. (B) Quantification of NOTCH1 expression within DP TCRβ^low cells of TAM-treated wild-type (n = 4) and lck-ER^T2-SCL (n = 4) mice. The proportion of total NOTCH1- and NOTCH1^intermediate-expressing cells was determined and quantified by the gating strategy shown in panel A. *P < .01; **P < .001. (C) Analysis of NOTCH1 expression within the abnormal CD8⁺TCRβ^low population of TAM-treated lck-ER^T2-SCL mice. (D) Flow cytometric analysis of a primary lck-ER^T2-SCL tumor (after 11 months of TAM treatment) and the corresponding cell line. Cells were stained for CD4, CD8, and NOTCH1. Dashed lines represent isotype control staining. (A,C,D) Numbers on the plots represent precentages of cells.