The study objectives will be to further delineate the spectrum of phenotypes associated with these enzymopathies, query for genotype/enzymatic/phenotype correlations and search for new genes in rare families that have evidence for an unknown class of methylmalonic acidemia and/or homocysteinemia. The population will consist of patients previously evaluated at NIH, physician referrals, and families directed to the study from clinicaltrials.gov as well as the Organic Acidemia Association. Most patients will be evaluated only at the NIH Clinical Center. However, if the NIH team decides that a patient under the age of 2 years is a candidate subject for this research protocol, that patient may enroll at the Children s National Medical Center (CNMC) site, pending approval by Dr Chapman, the Principal Investigator of the CNMC location. Outcome measures will largely be descriptive and encompass correlations between clinical, biochemical and molecular parameters.