Generation of dominant-negative β1 integrin transgenic mice (dnβ1). (a) The hprt targeting system used in our study: the dnβ1 transgene (comprising the IL2Rα extracellular and transmembrane domains and the β1 integrin cytoplasmic domain) and the MBP promoter were inserted by homologous recombination in single copy into the disrupted hprt locus on the X-chromosome of the hprt (−) ES cell line. The targeting vector contains the missing hprt sequences and thus restores hprt functionality and enables subsequent growth of the ES cells in HAT medium. (b) Transfected hprt (−) ES cells bearing a functional hprt gene (after successful recombination) were selected for HAT medium and screened by PCR using the primers represented as arrows in panel a. A band of 290 bp indicates the presence of the dnβ1 transgene. Twelve out of the twelve HAT⁺ clones correctly recombined. (c) Genotyping of transgenic mice was performed by PCR for wild-type (Wt) and dnβ1 transgenic alleles (dnβ1). Genotype of the four animals shown: Wt, wild-type; Hem, hemizygous male; Het, heterozygous. Panel a was adapted from Bronson et al. (1996).