POLR3B and leukodystrophy: Mutations in POLR3A, POLR3B, and POLR1C, encoding subunits of the ubiquitous RNA polymerase III (Pol III),4 cause the second most common form of childhood-onset hypomyelinating leukodystrophy (HLD) (MIM no. 607694, no. 614381, and no. 616494) (1, –, 3), a type of inherited neurodegenerative disorder characterized by deficient cerebral myelin formation (4).