Genetic diseases in humans are caused by mutations. Often these diseases are the result of a problem in metabolism, in particular, the lack of an enzyme used in one step of a metabolic pathway. For example, galactosemia is a genetic disease that results from the inability to completely breakdown the sugar lactose. Individuals with the disease lack a functional enzyme 3 in this metabolic pathway. On the basis of this type of information, and other evidence, the one gene-one protein hypothesis has been proposed. It states that the function of a gene is to provide the information to make a particular protein, a polypeptide, that will be used as an enzyme or a structural protein.