One of three alleles produced using a targeting construct capable of inducing different allelic variants when mated to mice that express cre, flp, or both. Referred to in the literature as Fgf8^delta2,3n, this allele lacks exons 2 and 3 and is a presumed null. Heterozygotes are normal. Homozygotes are smaller than littermates and lack all embryonic mesoderm- derived structures including heart and somites. This mutation causes lethality by embryonic day 10.5 (J:45909).