Ultra-orphan diseases: a quantitative analysis of the natural history of molybdenum cofactor deficiency.

Molybdenum cofactor deficiency has its onset during the neonatal period and infancy. There is considerable diagnostic delay. Although seizures were the most frequent initial cardinal sign, molybdenum cofactor deficiency should be considered as a differential diagnosis in patients presenting with hypotonia, developmental delay, or feeding difficulties. The survival data will inform further natural-history and therapeutic studies.