In human, TBX5 mutation causes Holt–Oram syndrome (HOS) which is associaited with upper limb abnormalities (Basson et al., 1997; Bongers et al., 2004), while TBX4 mutation results in small patella syndrome, which is characterized by foot dysplasia (Bongers et al., 2004) (Figure 1C). The gene discussed is TBX4; the disease is Holt-Oram syndrome.