This enzyme is one of the enzymes that play a role in the urea cycle. The lack of ASL results in excessive accumulation of nitrogen, in the form of ammonia (hyperammonemia), in the blood. Ammonia is a neurotoxin, which means that it damages or inhibits the function of neurons, the cells of the central nervous system. Excess ammonia travels to the central nervous system through the blood, resulting in the symptoms and physical findings associated with the disorder. Estimates are that ASA affects roughly 1 in 70,000 live births. The onset of symptoms usually occurs at birth but may not be noticeable for days or weeks. In some children, the onset of symptoms may not occur until later during infancy or childhood.