One of the most rewarding examples for teaching hereditary metabolic disorders is classical phenylketonuria (PKU) caused by the deficient function of phenylalanine hydroxylase, the locus of which (PAH) is on the long arm of the twelfth chromosome. The twelfth chromosome has also the locus (VWF, F8VWF) the pathogenic alleles of which cause impaired blood clotting--Willebrand's disease and it is at the same time also the site of the family of keratin genes (KRT) responsible for epidermolysis bullosa simplex and other diseases. The question of the relationship between membrane glucose transmitters--GLUT and diabetes (NIDDM) is the subject of many investigations concerned with these loci.