Combination of genetics and environment appear to account for the vast majority of cases of CAD. First-degree relatives of CAD patients are at higher risk of developing the disease and developing it earlier than the general population. Over 250 different genes have been implicated in the onset of CAD, making it a prime example of the combination of multiple genes and environment seen in complex disease. Defects in genes involved with LDL metabolism, homocysteine metabolism, muscle development, and blood pressure regulation have been associated. Other genes that have been linked encode the apolipoprotein A1 allele, apolipoprotein E4 allele, and glycoprotein IIb/IIIa. Familial hypercholesterolemia is due to a defective low-density lipoprotein receptor and is inherited in an autosomal dominant pattern. LDL levels elevated to values double that of normal are seen in affected persons as early as 2 years of age, and signs of CAD can be found by age 30.