Generation of CD44-deficient mice. (A) Restriction map of the CD44 genome fragment used to generate the targeting vector. Open boxes represent exons 2 and 3. 5′ flanking probe is designated as probe. Targeting vector replaces 3′ end of exon 2 and the entire exon 3 with a neomycin resistance gene in an antisense orientation containing several stop codons for all reading frames. Map of predicted homologous hybridization results in a 3.9-kb gene-targeted fragment compared with a 13-kb wild-type Kpn I fragment in C57BL/6J. (Abbreviations: N, Nco I; X, Xba I; K, Kpn I; K, e, Kpn I restriction sites in 129J mouse strain (missing in C57BL/6J mice), K, b, Kpn I restriction sites in C57BL/6 mice.) (B) Southern blot of homozygous-, heterozygous-mutant, and wild-type mice. Genotypes are indicated at the top of each lane: +/+, wild-type; +/−, heterozygous mutant; and −/−, homozygous mutant.