TBX5 and Tetralogy of Fallot: However, humans with TBX5 mutations also have other cardiac defects including tetralogy of Fallot (ToF), and rarely hypoplastic left heart syndrome (HLHS) (Basson et al. 1994; Newbury-Ecob et al. 1996), but these defects are not observed in Tbx5 heterozygous mice.