POLR3B and leukodystrophy: RNA polymerase III-related hypomyelinating leukodystrophy (POLR3-HLD; MIM: 607694, 614381, 616494), one of the most common hypomyelinating leukodystrophies, is an autosomal recessive disorder caused by biallelic pathogenic variants in POLR3A, POLR3B, POLR1C or POLR3K, each encoding subunits of RNA polymerase III.4–10POLR3A and POLR3B encode the largest subunits that form the catalytic core of RNA polymerase III.