Characterization of CBP homozygous mutant embryos. (A) RT-PCR analysis of RNAs isolated from yolk sacs of wild-type embryos, heterozygous embryos, and homozygous embryos. A CBP 5′-end forward primer and a 3′-end reverse primer, indicated in Fig 1A, are used to detect wild-type CBP transcripts. The same CBP 5′ forward primer and a reverse primer in the splice acceptor region are used to detect CBP/β-geo fusion transcripts. (B) Western blotting of extracts (30 μg protein) from the brains of adult mice (right) and extracts (5 μg protein) from E9.5 embryos (left) with N-terminal specific anti-CBP antibodies (CBP-A22). An arrowhead indicates the 265-kD wild-type CBP protein. An arrow indicates the truncated form of CBP at 121 kD. (C) The schematic structure of putative truncated and wild-type CBP and p300. Percentages refer to amino acid (aa) identify between proteins. (D) Phenotypes of wild-type and CBP homozygous mutant embryos at E9.5. A mutant is smaller and much paler than a wild-type littermate. The morphology of the cranial region (arrow) of mutants is distinct from that of wild-type littermates. (E) A CBP homozygous mutant displays an open neural tube (arrowhead) in the cranial region.