Urea cycle abnormalities affect around one in every 30,000 births. Disorders of the urea cycle are inherited. Genes guide the body on how to break down protein. We have two copies of each gene, and most UCDs develop when a person gets a modified gene from both parents. This means that they share one copy of each genetic marker, or allele, for that matter. People with UCD cannot break down ammonia into urea. So instead of removing toxic substances from the body like other cells can, these special cells accumulate toxins that can cause serious problems if not treated. Toxins include excess amounts of ammonia, which can be dangerous if not removed from the body; phenylalanine, which can cause nerve damage and mental retardation if not removed from the blood stream; and nitric oxide, which can block blood vessels and cause stroke or heart attack if not released from muscle tissue. People who inherit UCD may experience symptoms during pregnancy or soon after birth because their bodies are not producing enough enzymes needed to process protein. Without these enzymes, abnormal proteins build up in tissues and organs across the body. These defects can lead to liver disease, developmental delays, coma, and death. Children born with UCD usually appear healthy at first but often develop symptoms later in life.