Tyrosinemia type 3 is a genetic disorder characterized by elevated blood levels of the amino acid tyrosine, a building block of most proteins. This condition is caused by a deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase, one of the enzymes required for the multi-step process that breaks down tyrosine. This enzyme shortage is caused by mutations in the HPD gene. Characteristic features include intellectual disability, seizures, and periodic loss of balance and coordination (intermittent ataxia). Tyrosinemia type 3 is inherited in an autosomal recessive manner.