Dihyropyrimidine dehydrogenase (DPYD) deficiency is an inherited disorder with large variability. It is due to the deficiency of the dihydropyrimidine dehydrogenase enzyme. Individuals may be severely affected or show no symptoms at all. Symptoms for those severely affected begin in infancy and include small head size, increased muscle tone, recurrent seizures and intellectual disability. Unfortunately, at this time, it cannot be determined who will show symptoms and who will not. Currently there is no cure for DPYD deficiency, but medical surveillance and care may help to improve some symptoms and overall condition of life. In addition, those who are severely affected, as well as those with no symptoms can develop severe toxicity after the administration of the antineoplastic drug 5-fluorouracil (5-FU), a commonly used medication for chemotherapy of many types of cancers. 5-FU has also been given topically for actinic keratoses and Bowen's disease. DPYD deficiency is caused by pathogenic variants in the DPYD gene.