PIGG deficiency is caused by the mutation of PIGG, an enzyme which transfers ethanolamine phosphate (EtNP) to the second mannose in the GPI biosynthesis. This EtNP is removed by PGAP5 after GPI is attached to the precursor protein in the ER. Thus, the PIGG deficient cells express the GPI-anchored proteins at normal level with the normal structure on the cell surface. However, the affected individuals with PIGG deficiency shows neurological abnormalities, suggesting that this transient modification is important for the neurological development. The main clinical features are intellectual disability, developmental delay, epilepsy, hypotonia and cerebellar abnormalities. The prevalence of PIGG deficiency is unknown as it is recently recognized disease, but there have been 23 families reported. The PIGG gene is located on chromosome 4p16.3, and PIGG deficiency is autosomal recessive disorder.