Earliest cases of biochemical mutations were described in man by A.E. Garrod in 1909 in his book “Inborn Errors of Metabolism”. There are three important diseases associated with metabolic breakdown of phenylalanine (Fig. 22.2). (i) Phenylketonuria is due to accumulation of phenylpyruvic acid and causes mental disorders. Children suffering with this disease are known as phenylpyruvic idiots and are unable to breakdown phenylpyruvic acid into hydroxy-phenylpyruvic acid, (ii) Alcaptonuria is due to lack of ability to breakdown homogentisic acid into acetoacetic acid. Due to accumulation of homogentisic acid, the urine of patients suffering with this disease turns black as soon as it comes in contact with air. (iii) Albinism is due to absence of melanin pigment and individuals suffering with this disease are incapable of converting dihydroxy-phenylalanine into melanin. Another disease tyrosinosis is also associated with the same metabolic pathway (Fig. 22.2).