Inactivation of the murine CD24a (HSA) gene. (A) The promoter and first exon of the CD24a gene was replaced by a neomycin-resistance expression cassette in mouse embryonal stem cells by homologous recombination. (B) A Southern blot of EcoRI digested tail DNA was hybridized with a HincII-BamHI genomic DNA probe derived from the region immediately 5′ to the targeted replacement. Mice heterozygous for the mutation (+/−) show bands from both the intact allele (upper band) and the targeted allele. (C) Northern blots of total RNA extracted from the spleen and liver of both wild type (+/+) and HSA-deficient mice (-/-) was sequentially hybridized with HSA and glyceraldehyde phosphate dehydrogenase cDNA probes.