Genome-wide association studies of TD and Tourette syndrome demonstrate high polygenicity and have identified a few genome-wide significant signals, including Collagen Type XXVII Alpha 1 chain, COL27A1 and Fms Related Receptor Tyrosine Kinase 3, FLT-3, though neither locus has been replicated in an independent study to date [30–32]. This evidence concerns the gene COL27A1 and thanatophoric dysplasia.