Mice that are heterozygous for the targeted allele are viable and fertile. Mice homozygous for this mutation have a lethal phenotype. Half of homozygous null mice have an embryonic lethal phenotype, dying between 10.5 to 12.5 days post coitum. The cause of embryonic death may be due to circulatory abnormalities. Embryonic death also occurred in late gestation, with the remaining homozygous mutants dying at birth, due to respiratory failure. At 12.5 dpc, initial cartilaginous primordia of mutant embryos forelimbs are not abnormal, but by 13.5 dpc, mutant embryos display severe foreshortening of the forelimbs. At birth, the length of long bones of mutant animals are only one third the length of long bones of wildtype animals. Mutants have reduced chondrocyte proliferation, abnormal chondrocyte maturation and absence of mature osteoblasts in endochondral bones. Recent studies have linked mutations of Ihh to brachydactyly type A-1 St-Jacques. This mutant mouse strain represents a model that may be useful in studies of skeletal morphogenesis. The Jackson Laboratory 2024, used with permission.