Silkie the "SHH" gene, called the ZPA Regulatory Sequence (ZRS). This causes ectopic "SHH" expression in the anterior of the developing limb bud, leading to increased tissue growth and digits. While the feet of the Silkie display polydactyly, the wings have the standard tridactyly (three digit) arrangement. The Japanese Silkie initially develops additional digits in the wing as an embryo, but these are lost prior to hatching. The genetic cause behind Silkie polydactyly differs from those that cause polydactyly in the Dorking chicken breed, which is due to ectopic "FGF4" expression in the AER, with ectopic "SHH" a secondary effect. The
The Silkie (novel) explain the origin of the Silkies as an experiment in genetic manipulation, the action moves forward over a hundred years to a future in which the Silkies are numerous and live on Earth. Humanity has assimilated them by means of the Special People, who can establish telepathic rapport with the Silkies. All Silkies are male, and most are married to women of the Special People. They are employed as police in space, and most are comfortable with that role. One dedicated Silkie, Nat Cemp, encounters three different alien races, and with each encounter he gains more powers and learns more