Congenital disorder of glycosylation (CDG) is a group of inherited disorders caused by defects in glycosylation, the process of attaching sugars to proteins and lipids. The most common form of CDG is type IA, which is due to a deficiency in an enzyme named phosphomannomutase 2 and is sometimes referred to as phosphomannomutase 2 deficiency. CDG-IA affects the nervous system and many other organs and systems. Symptoms are highly variable, ranging from severe infantile multisystem involvement to mildly affected adults. Infants will typically have symptoms that include developmental delay, feeding difficulties, weak muscle tone, inverted nipples, and an abnormal distribution of fat. About 20 percent do not survive the first year of life due to severe infections or organ failure. The most severe form of CDG-IA appears in pregnancy, causing extreme fluid build-up in the body before birth. Babies are usually stillborn or die soon after birth. Individuals who survived infancy may have intellectual disability, inability to walk, joint and spine deformities, stroke like episodes, and absence of puberty for females. Currently there is no standard treatment for individuals with CDG-IA, but medical surveillance and care may help to improve some symptoms and overall condition of life. CDG-IA is caused by pathogenic variants in the PMM2 gene.