Inherited defects in the gene for the low density lipoprotein (LDL)-receptor give rise to familial hypercholesterolaemia (FH), a disorder in which defective catabolism of LDL causes a marked increase in its concentration in plasma. As a result, there is excessive deposition of cholesterol in the arterial wall leading to accelerated atherosclerosis and premature coronary heart disease in most patients, although there are differences in its severity. Many different mutations have been found in the LDL receptor genes of FH patients, and although this heterogeneity has provided information about the relationship between structure and function in different domains of the protein, it makes simple DNA-based diagnosis of the disease impossible. When sufficient groups of patients with defined mutations are available it will be possible to determine the relative importance of any particular mutation compared with other genetic or environmental factors in relation to the severity of their symptoms or their response to treatment.