While it is known that mutations in HOXA13 or HOXD13 can cause specific limb phenotypes, such as hand-foot-genital-syndrome [42], [43] or synpolydactyly [44], [45], no HOXA9-related disease phenotype has been identified in patients so far, leaving the role of HOXA9 during human limb development unclear. This evidence concerns the gene HOXD13 and synpolydactyly.