POLR3B and leukodystrophy: Pol III is a 17-subunit complex responsible for transcribing several different noncoding RNAs, and of its subunit-encoding genes, variants in POLR3A and POLR3B were first discovered to cause POLR3-HLD, followed by POLR1C and, more recently, POLR3K and POLR3D [1–7].