However, poly-A repeats in HOXD13, the causative mechanism of a non-neurological condition, synpolydactyly, represents an exception, in which the most likely mechanism leading to increased poly-A tracts are errors in DNA replication (Muragaki et al., 1996; Warren, 1997; Brown and Brown, 2004). The gene discussed is HOXD13; the disease is Syndactyly type 2.