RAB39B and cerebral creatine deficiency syndrome: Specific conditions and their associated genes in Xq27-28 that require investigation include FRAXE syndrome- FMR2, Rett syndrome-MECP2, X-linked Mental retardation-41-GDI1, Creatine deficiency syndrome-SLC6A8, X-linked mental retardation-72-RAB39B and X-linked adrenoleukodystrophy-ABCD1.