Carbohydrate-deficient glycoprotein syndrome type I (CDGI) is most often due to phosphomannomutase deficiency; paradoxically, the human phosphomannomutase gene PMM1 is located on chromosome 22, whereas the CDGI locus is on chromosome 16.
It is especially striking that Pmm1, despite an almost complete overlap of its expression with Pmm2, e.g., in the developing brain, is apparently unable to compensate for deficient Pmm2 activity in CDG-Ia patients.