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.