Notch1-deficient cells continue to express Notch3. Thymocytes from mice carrying a conditionally deleted allele of Notch1 (Lck[Cre]^+/−Notch1^fl/fl dashed line), control mice (Lck[Cre] only, solid line) were stained using an Ab specific for Notch3. No reduction was found in the levels of surface-expressed Notch3 in thymocytes lacking Notch1, even in pre-DP cells, by which stage Lck[Cre]-mediated deletion is long since finished. These findings show that Notch3 is not regulated by Notch1, and thus the absence of a more severe thymic phenotype in Notch1/3 double mutant mice (Figure 5) is not because of the absence of Notch3 in Notch1-mutant mice. Filled histograms indicate staining of a nonspecific isotype-matched control Ab on control cells.