POLR3B and leukodystrophy: RNA polymerase III (POLR3)-related leukodystrophy or 4H (Hypomyelination, Hypodontia and Hypogonadotropic Hypogonadism) leukodystrophy (MIM 607694, 614381)3 was found to be caused by recessive mutations in POLR3A (MIM 614258) or POLR3B (MIM 614366)4, 5, 6, 7, 8, 9 and is characterized by an expanding spectrum of clinical3, 10 and radiological features10, 11, 12.