Biotinidase deficiency (McKusick 253260) leads to a progressive deficiency of the vitamin biotin, an essential cofactor for the carboxylation reactions of pyruvate, propionyl-CoA, 3-methylcrotonyl-CoA and acetyl-CoA. This disease usually presents with neurological symptoms such as hypotonia, convulsions and ataxia, while skin rash and alopecia usually appear later. If treatment is delayed, irreversible brain damage such as optic atrophy and neurosensory hearing loss may result. The neurological dysfunction appears to be more severe than that observed in holocarboxylase synthetase (HCS) deficiency, propionic acidaemia or isolated 3-methylcrotonyl-CoA carboxylase (MCC) deficiency