Assessing the Safety and Tolerability of NMN in DHDDS-CDG

Inclusion Criteria: * Subject is ≥ 4 years old * Subject has biologically and genetically proven heterozygous DHDDS-CDG. * Subject/legally authorized representative (LAR) is able to understand and provide written informed consent, and assent (as applicable) to participate in this study. Exclusion Criteria: * Subject has intellectual disability with IQ\<52 (moderate or lower IQ intellectual disability). * In the site Principal Investigator's opinion, subject has a history of intolerance to NMN or other niacin metabolite supplement that precludes the subject from participation in this study. * Subject has any of the following: * Liver failure * ALT level \>5x ULN * AST level \>5x ULN * eGFR \< 30 OR creatinine \>180 mmol/L * Subject is pregnant. * Use of investigational compounds within the previous 6 months or current enrollment in another trial involving investigational compounds. * Concomitant use of the following medications that could interact with orally administered NMN: * Aspirin * Metformin * Statins or other cholesterol-lowering drugs * In the site Principal Investigator's opinion, subject is not able or willing to comply with the trial requirements.