Thymocytes from mice with a gene-trapped Notch3 allele exhibit slightly reduced cellularity, but otherwise develop normally. (A) Absolute cell counts in thymuses from mice homozygous for a targeted (lacZ gene trapped) Notch3 allele, or wild-type control mice; a ∼ 15% reduction P < .005, Student 2-tailed t test for independent samples) in cellularity is seen in Notch3^lacZ/lacZ mice (n = 15 for each type of mice). Nonetheless, the overall proportion of cells at each developmental stage, as defined by CD4 versus CD8 (B), or CD117 versus CD25 on lineage-negative cells (C) was not different in targeted versus control mice. The frequency of cells in the TCR-γδ or NK lineages was also nearly identical in Notch3^lacZ/lacZ and control mice (D). (E-F) The phenotype of cells developing in the thymus of competitive chimeras constructed from N3^lacZ/lacZ or wild-type donor cells mixed with allelically marked (CD45.1⁺) donor cells. Again, no substantial differences in phenotype were noted.