HELLS and ICF syndrome: Even though pericentromeric repeats and PCDH (protocadherin) genes are equally hypomethylated in all ICF syndrome patients, ZBTB24, CDCA7, and HELLS mutations mainly affect heterochromatic CpG-poor regions (open seas), some of which are associated with genes related to neuronal development, which is consistent with the more pronounced mental retardation in these subtypes of patients [166].