In agreement with the present study, all the Spanish patients studied (belonging to a cohort of 33 MSUD patients) by Rodríguez-Pombo et al. [9] who had a mutation in the BCKDHB gene (subtype Ib) also had the classical phenotype; however, inconsistencies were found between the biochemical parameters of the patients and their clinical phenotype, as in the case of three patients who were homozygotic for the same mutation and exhibited a residual enzymatic activity variable between < 1 and 13%. The gene discussed is BCKDHB; the disease is maple syrup urine disease.