In particular, molecular analysis was critical in patient ID 048 that carried a HCFC1 gene mutation of uncertain significance and showed persistently increased C3 and modest methylmalonic acid urinary excretion; in this case the combination of biochemical and molecular data suggested a diagnosis of X-linked methylmalonic aciduria and homocystinuria, CblX type, likely with a mild presentation, and indicated further monitoring of the patient. Here, HCFC1 is linked to homocystinuria.