Identification of a disease causing gene rarely leads to the invention of a novel therapy. Even in the case of genetic disorders or diseases greatly influenced by a hereditary component, knowledge about the genotype does not necessarily result in the invention of a novel therapy. In many cases effective treatment was introduced earlier without any existing knowledge of the molecular basis of the disease, or even without a full recognition of the pathomechanisms. Phenylketonuria is the best known example. Great expectation are connected with experimental therapy of phenylketonuria: enzymatic therapy, blocking the brain-blood barrier for phenylalanine and novel forms of low phenylalanine diet. Molecular diagnostics could be helpful in identifying patients responsive to non-standard therapy, such as tetrahydrobipterin supplementation. Novel research and development strategies will contribute to a reduction of the negative aspects of present treatment and they will facilitate normal physical a intellectual development of children with phenylketonuria, as well as to help patients in their everyday life.