Generation of eotaxin null mice. (A) Strategy for generating eotaxin knockout mice by homologous recombination. The three-exon and two-intron genomic structure of the eotaxingene is shown on the top. The targeting vector is shown in the middle. The 5′ arm contains 3.2 kb of mouse genomic DNA with the sequence encoding the first 4 amino acids of eotaxin fused in-frame to the E coli β-galactosidase gene. The 3′ arm of 3.5 kb of mouse genomic DNA containing exon 3 and the 3′ untranslated sequence of the eotaxin gene is inserted between the neomycin-resistance gene (neo) and the herpes simplex virus thymidine kinase (tk) gene. The targeted allele after homologous recombination and the screening strategy are shown at the bottom. The black boxes represent the exons, with their respective number above. Southern blot analysis of SpeI-digested genomic DNA from ES cells (B) and from wild-type (+/+), heterozygous (+/−), and knockout (−/−) mice (C). The upper band (14 kb) shows the wild-type allele, and the lower band (10 kb) shows the targeted allele. (D) Northern blot of total RNA (10 μg) from lymph nodes of wild-type (+/+), heterozygous (+/−), and knockout (−/−) mice. The blot was probed with³²P-labeled mouse eotaxin cDNA (upper panel), stripped, and tested with a probe specific for mouse GAPDH (bottom).