Critically, a large cohort of patients with hypomyelinating leukodystrophies have mutations mapping to factors implicated in the ribosome/tRNA axis:35 for example, mutations in the translation factor eIF2B cause Vanishing White Matter disease36, and mutations in RNA polymerase III subunits POLR3A and POLR3B, required for tRNA transcription, cause 4H Leukodystrophy or POLR3-Related Leukodystrophy, respectively37–41. Here, POLR3B is linked to leukodystrophy.