(a) Targeted disruption of the H3 allele in mice. The mouse H3 gene, targeting construct, and expected targeted allele are shown. Each of the three exons is indicated by black boxes. Horizontal arrows show the size of the expected fragments after BamHI digestion. Two probes that were used in Southern blot analysis are shown. B, BamHI; S, SmaI; Xb, XbaI. (b) Southern blot analysis of genomic DNA from mouse tail and hybridization after digestion of DNA with BamHI using probe 1 from (a). +/+, H3^+/+; +/–, H3^+/–; –/–, H3^–/–. (c) Northern blot analysis of brain total RNA. The H3 exon 3 probe was used in this experiment, and GAPDH was used as an internal control. (d) Agonist binding assay of brain membranes with [³H]NAMHA. Data are representative of three experiments. H3^+/+ mice are indicated by squares and H3^–/– mice by triangles. (e) Northern blot analysis of brain mRNA. H1- and H2-specific probes were used, and GAPDH served as an internal control.