Some RNA polymerase (POLR) 3-related leukodystrophy cases do not have the causal mutations in POLR3A and POLR3B. Here, by exome sequencing, the authors identify recessive mutations in POLR1C, a gene encoding a shared POLR1 and POLR3 subunit, impairing assembly and nuclear import of POLR3, but not POLR1. Here, POLR3B is linked to leukodystrophy.