1. ^* Howard Hughes Medical Institute, Department of Human Genetics, University of Utah School of Medicine, Salt Lake City, Utah 84112, USA
2. ^† Division of Basic Science Research and Pathology, Children's Hospital Research Foundation and Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, Ohio 45229, USA
3. ^‡ To whom correspondence should be addressed

Abstract

MICE with targeted disruptions¹ in Hoxgenes have been generated to evaluate the role of the Hox complex in determining the mammalian body plan. This complex of 38 genes encodes transcription factors that specify regional information along the embryonic axes. Early in vertebrate evolution an ancestral complex shared with invertebrates was duplicated twice to give rise to the four linkage groups (Hox A, B, C and D)^2,3. As a consequence, corresponding genes on the separate linkage groups, called paralogues, are most closely related to each other. Based on sequence similarities, the Hox genes have been subdivided into 13 paralogous groups. The five most 5' groups (Hox9–13) pattern the posterior region of the vertebrate embryo and the appendicular skeleton^4–18. Mice with individual mutations in the paralogous genes hoxa-11 and hoxd-11 have been described^15–18. By breeding these two strains together we have generated double mutants which have dramatic phenotypes not apparent in mice homozygous for the individual mutations. The radius and the ulna of the forelimb are almost entirely eliminated, the axial skeleton shows homeotic transformations, and there are severe kidney defects not present in either single mutant. The limb and axial phenotypes are quantitative: as more mutant alleles are added to the genotype, the phenotype becomes progressively more severe. The appendicular skeleton defects suggest that paralogous Hoxgenes function together to specify limb outgrowth and patterning along the proximodistal axis.