A Outline of human heart development. The X axis displays days of human and mouse gestation. B The spectrum of congenital heart disease from mild to severe. The lesions indicated as “severe” are expected to require intervention in the first year of life. ASD-Atrial Septal Defect, VSD-Ventricular Septal Defect, CoA- Coarctation of the Aorta, TOF-Tetralogy of Fallot, TGA- Transposition of the Great Arteries, HLHS- Hypoplastic Left Heart Syndrome. Classes of CHD based on proposed developmental-genetic mechanisms are indicated in parentheses; LVO-Left Ventricular Outflow Obstruction, CTD- Cono-Truncal Defect, HTX- Heterotaxy. C Genetic causes of CHD identified to date.