Familial defective apo B-100 is an autosomal dominant trait which causes moderate to severe elevation of LDL-cholesterol in the plasma and may predispose the coronary heart disease. The primary defect is a single amino acid change (Arg3500 to Gln), which disrupts the normal binding of LDL to the LDL-receptor. The mutation occurs at an estimated frequency of 1/600 in the normal population and is, therefore one of the most common known single-gene defects causing an inherited abnormality.