That up to 20% of Sudden Infant Death Syndrome cases may be attributed to an undiagnosed inborn error of metabolism such a urea cycle disorder. Urea cycle disorders are included in the category of inborn errors of metabolism. There is no cure. Inborn errors of metabolism represent a substantial cause of brain damage and death among newborns and infants. Because many cases of urea cycle disorders remain undiagnosed and/or infants born with the disorders die without a definitive diagnosis, the exact incidence of these cases is unknown and underestimated. It is believed that up to 20% of Sudden Infant Death Syndrome cases may be attributed to an undiagnosed inborn error of metabolism such a urea cycle disorder. In April 2000, research experts at the Urea Cycle Consensus Conference estimated the incidence of the disorders at 1 in 10,000 births. This represents a significant increase in case identification and diagnosis in the last few years.  Research studies have now been initiated to more accurately determine the incidence and prevalence of UCDs

Urea cycle disorder

A urea cycle disorder is a genetic disorder caused by a deficiency of one of the six enzymes in the urea cycle which is responsible for removing ammonia from the blood stream. The urea cycle involves a series of biochemical steps in which nitrogen, a waste product of protein metabolism, is removed from the blood and converted to urea. Normally, the urea is transferred into the urine and removed from the body. In urea cycle disorders, the nitrogen accumulates in the form of ammonia, a highly toxic substance, and is not removed from the body resulting in hyperammonemia (elevated blood ammonia). Ammonia then reaches the brain through the blood, where it causes irreversible brain damage, coma and/or death.

source=The National Urea Cycle Disorders Foundation 

California ( usa)

Dr Archibald Garrod was the first to connect a human disorder with Mendel laws of inheritance. He also proposed the idea that diseases came about through a metabolic route leading to the molecular basis of inheritance
Dr Archibald Garrod was the son of the physician, Alfred Baring Garrod, who diagnosed and studied rheumatoid arthritis. Although his father initially intended for Archibald to study business, his teachers recognized and encouraged him to go into the field of science and medicine. Dr Garrod studied medicine at Oxford University and became a physician
Dr Garrod was studying the human disorder alkaptonuria. He collected family history information (as well as urine) from his patients. Based on discussions with Mendel advocate William Bateson, Dr Garrod deduced that alkaptonuria is a recessive disorder. In 1902,Dr Garrod published a book called The Incidence of Alkaptonuria: a Study in Chemical Individuality. This is the first published account of a case of recessive inheritance in humans.

Dr Garrod was also the first to propose the idea that diseases were "inborn errors of metabolism." He believed that diseases were the result of missing or false steps in the body chemical pathways. In 1923, his studies on alkaptonuria, cystinuria, pentosuria, and albinism were published as a book: Inborn Errors of Metabolism.Dr Garrod attributed a biochemical role to genes, and laid the groundwork for the next wave of discovery — the molecular basis of inheritance.
Source=http://www.dnaftb.org/13/bio.html