Give me five! Or six? Or seven?

Give me five, high five, hello and good bye - all shown to others with one hand. Certainly everyone knows that this hand comprises five digits. But not everyone is born with exactly five digits. In humans, occasionally hereditary skeletal limb malformations do occur. One malformation, the so-called synpolydactyly, implies that patients are born with additional digits, and those are fused above all. This phenotype develops due to a mutation within the Hoxd13 gene, leading to a lack of retinoic acid and thereby causing uncontrolled cartilage production at wrong sites in the extremity.