Four types of rare diseases

Rett syndrome, originally termed cerebroatrophic hyperammonemia, is a genetic postnatal neurological disorder of the gray matter of the brain that almost exclusively affects females but has also been found in male patients. The clinical features include small hands and feet and a deceleration of the rate of head growth. Patients typically have no verbal skills and about 50 percent of individuals affected do not walk. Repetitive stereotyped hand movements, such as wringing and/or repeatedly putting hands into the mouth, are also noted.

Phenylketonuria (PKU) is a metabolic genetic disorder characterized by mutations in the gene for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate (also known as phenylketone), which can be detected in the urine. Untreated PKU can lead to intellectual disability, seizures and other serious medical problems.

Osteogenesis imperfecta (OI and sometimes known as brittle bone disease) is a congenital bone disorder characterized by brittle bones that are prone to fracture. People with OI are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of Type-I collagen. Eight types of OI can be distinguished. Most cases are caused by mutations in the COL1A1 and COL1A2 genes.