Two novel rare mutations, MCAD 842GfiC (R256T) and MCAD 1166AfiG (K364R), have been investigated to assess how far the bio-chemical properties of the mutant proteins correlate with the clinical phenotype of medium chain acyl-CoA dehydrogenase (MCAD) deficiency. When the gene for K364R was overexpressed inEscherichia coli, the syn-thesized mutant protein only exhibited activity when the gene for chapero-nin GroELS was co-overexpressed.