Recent studies on galactosaemia, phenylketonuria and homocystinuria.

Galactosaemia, phenylketonuria and homocystinuria are inborn errors of metabolism as the term was defined by Garrod (1908). I n all three lack of an enzyme causes a metabolic block and accumulation of the substrate of the missing enzyme, and all three are inherited as Mendelian recessive characters. Unlike Garrod's original examples, galactosaemia, phenylketonuria and homocystinuria are often accompanied by damage to the central nervous system and the majority of those affected are mentally retarded (Woolf, 1962a, 1963 ; Gerritsen & Waisman, 1966).