HOXD13 and synpolydactyly: Missense variants affecting other residues of the C-terminal HOXD13 homeodomain may also give rise to different and more severe limb phenotypes such as synpolydactyly (SPD; MIM#186000) or syndactyly type 5 (MIM#186300), whereas expansion or contraction of the N-terminal HOXD13 polyalanine tract usually results in SPD [3-6].