Publisher Summary This chapter focuses on expression and phenotype of the Hoxd-3. The anterior limit of Hoxd3 expression lies at the boundary between rhombomere four and rhombomere five in the developing nervous system, and the boundary between somite four and five in the paraxial mesoderm. This chapter presents a study in which most Hoxd-3 homozygous mutants died within a few days of birth although a few have survived to adulthood and were fertile. Alterations in the structures of the cervical vertebrae in all mutants may have led to accidental cervical dislocation. The anterior arch of the atlas was transformed into an extension of the basioccipital bone of the skull. The lateral masses of the atlas were also altered and showed variable extents of fusion with the exoccipitals. The axis showed atlas-like characteristics.