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. CDG type Ib, which is also called MPI-CDG, is due to reduced activity of an enzyme called phosphomannose isomerase. CDG-Ib is distinct from most other CDGs in two ways. First, CDG-Ib does not significantly affect the central nervous system. Second, this potentially fatal disorder can be treated. CDG-Ib affects the liver and the gastrointestinal tract. Infants with CDG-Ib present with chronic diarrhea, recurring vomiting, liver problems, low blood sugar, and malnutrition. Severity is variable among affected individuals. CDG-Ib can be treated effectively by lifelong dietary supplements of a particular sugar known as mannose. CDG-Ib is caused by pathogenic variants in the gene MPI.