Patients of any gender, ethnicity, and age are eligible to enroll in this protocol. Affected individuals under the age of 2 years may be clinically evaluated at Children s National Medical Center (CNMC) as part of an evolving agreement in the Translational Program in Pediatrics, if they are deemed eligible for participation. Patients will be diagnosed based on a determination of MMA and homocysteine levels in plasma and urine. Most will have their complementation class known or pending. Some patients who have not yet had this laboratory test will be admitted to the protocol based upon metabolic parameters and clinical history. This latter category of patients might include individuals with a suspected genetic but unknown class of MMA. We will also obtain specimens for patients under 2 years old with a suspected diagnosis of combined malonic and methylmalonic aciduria (CMAMMA) for ACSF3 genetic testing because the NIH is the only center offering CLIA certified molecular diagnosis.