Multiple carboxylase deficiency (MCD) is an autosomal recessive metabolic disorder characterized primarily by cutaneous and neurologic abnormalities. Symptoms result from the patient's inability to reutilize biotin, a necessary nutrient. Sweetman (1981) recognized that multiple carboxylase deficiency could be classified into early (see 253270) and late forms. The early form showed higher urinary excretion of 3-hydroxyisovaleric acid and 3-hydroxypropionic acid than the late form and was associated with normal plasma biotin concentrations. Sweetman (1981) proposed a defect in holocarboxylase synthetase and intestinal biotin absorption, respectively. Some patients with biotinidase deficiency present in infancy (Baumgartner et al., 1985; Kalayci et al., 1994), and some individuals with this deficiency are asymptomatic (Wolf et al., 1997).