Figure 2. Reduced thymic cellularity in thymus-initiated Cre-transgenic strains.

Each bar represents the mean ± s.d. for 6–13 individual experiments (thymuses); mice were males and females, 5 weeks of age. Statistics represent the Student's two-tailed t test for unpaired samples. In mice hemizygous for the a Cre allele, both Lck strains exhibited substantial reductions in thymic cellularity, while Cd2[Cre] and Cd4[Cre] did not. Mice that were homozygous for Cre alleles revealed a further exacerbation of this phenotype that was significant even in previously unaffected Cd4[Cre] mice (note: Cd2[Cre] homozygous mice were not generated, due to embryonic lethality). Since no loxP target sequences were present, these data suggest the existence of off-target effects of Cre in developing thymocytes.