There appear to be at least two underlying aetiologies for combined carboxylase deficiency; firstly, a failure of biotinylation of apocarboxylases due to a mutation of holocarboxylase synthetase (EC 6.3.4.10) which results in an enzyme with a high Km with respect to biotin and secondly, a failure of biotinylation due to a lowered availability of biotin due to biotinidase deficiency (EC 3.5.1.12). In both these disorders secondary defects of all four biotin-dependent carboxylases result which in turn causes the excretion of the metabolites characteristic of the isolated carboxylase deficiencies. In addition, both disorders respond biochemically and clinically to the administration of large amounts of biotin.