A PGK-neomycin resistance cassette replaced the endogenous promoter with the lck promoter, which is active in developing thymus. Northern blot analysis using a full length cDNA probe detected Dntt transcript in thymus of homozygous mutant E14.5-E18.5 embryos and in adults, as well as in homozygous mutant adult spleen, bone marrow, lymph node and lung, but not in mutant adult kidney, brain or liver. The Dntt expression pattern observed in the mutant mice was identical to that of the Lck gene in normal mice.