Many genetic disorders are caused by mutations in single genes (monogenic diseases), and the inheritance pattern of these diseases follows simple rules. If a mutation in both copies of the gene on both chromosome homologues is necessary to cause the disease, the inheritance pattern is recessive, and a patient is the offspring of two clinically unaffected carriers. However, if a mutation in a single homologue is sufficient, the inheritance pattern is dominant and the disease is transmitted from generation to generation. Monogenic diseases are responsible for only a small fraction of all patients with genetic diseases. Many common diseases, including cancer, heart disease, diabetes and several psychiatric diseases, are the results of a complex interaction between genetic and environmental factors. However, these diseases can have important genetic components and can therefore still be considered genetic diseases. The identification of genes involved in complex genetic diseases can be very important for the understanding and treatment of these diseases. Over the last 15 years, much progress has been made in the identification of genes responsible for monogenic diseases, but the identification of genes involved in complex diseases has been more difficult, and at this moment little is known about the genes involved in most common diseases.