3-phosphoglycerate dehydrogenase deficiency is an inherited disorder caused by the inability of the body to make serine, which is building block for protein. This is due to a dysfunctional enzyme called phosphoglycerate dehydrogenase. Affected infants typically are born with small heads or head sizes that progressively get smaller as the child ages. This is due to loss of brain cells and brain tissue as a result of the lack of serine for brain development. Affected children typically have seizures, severe delays in achieving developmental milestones and involuntary momentary loss of muscle use and coordination. They will also have intellectual disability. In some very rare cases, affected individuals will not show signs or symptoms of the disorder until adolescence or adulthood. There is no cure for 3-phosphoglycerate dehydrogenase deficiency and only in some cases has supplemental dietary serine had any positive effect. 3-phosphateglycerate dehydrogenase deficiency is caused by pathogenic variants in the PHGDH gene.