SP6 and Poland syndrome: Certain types of syndactyly have a different pathway of pathogenesis (such as Poland syndrome syndactyly) or the pathogenesis is yet to be determined (such as Shaker syndactyly mouse models, syndactyly related to b-HLHA9 in both humans and animals, syndactyly of Noggin-null mice, syndactyly of Sp6 mutant mice, and isolated 4/5 metacarpal fusion in humans).