POLR3B and leukodystrophy: Recessive mutations in POLR3A, POLR3B, POLR1C and POLR3K cause RNA Polymerase III-related hypomyelinating leukodystrophy (POLR3-HLD), [1–4] a devastating childhood-onset neurodegenerative disorder characterized by motor regression, cerebellar features and/or cognitive dysfunction, as well as hypomyelination and cerebellar atrophy on magnetic resonance imaging (MRI) [5].