The mouse Fused toes (Ft) mutation is the result of a 1.6-Mb deletion including the entire Iroquois B gene cluster.

As a result of transgenic insertional mutagenesis, heterozygous Fused toes (Ft) mice display a syndactyly of forelimbs and a thymic hyperplasia. Homozygous Ft/Ft embryos die in midgestation, exhibiting a deformation of craniofacial structures, a syndactyly and a polydactyly of fore- and hindlimbs, a disorganization of the ventral spinal cord, and defects in left-right axis formation. Here we report on our structural analyses of the Ft mutation. We established a physical as well as a gene map of the Ft locus, showing that the transgene integration resulted in a deletion of 1.6 Mb of genomic sequences on mouse Chromosome 8. Owing to this deletion, six genes, including the entire IroquoisB (IrxB) gene cluster, are directly affected by the Ft mutation.