Homozygous and compound heterozygous mutations affecting the genes encoding phosphatidylinositol glycan (GPI) anchor biosynthetic enzymes class V (PIGV, located on chromosome 1p36.11) and class O (PIGO, located on 9p13.3) cause two very rare CDGs that manifest with facial dysmorphism, skin abnormalities, mental retardation, epilepsy and gastrointestinal defects. This evidence concerns the gene PIGV and epilepsy.