Currently, the POLR3-HLD disease population is divided with the majority (≥90%) of patients having either biallelic mutations in POLR3A or POLR3B (Bernard et al., 2011; Tétreault et al., 2011; Daoud et al., 2013; Wolf et al., 2014b) and a minority (<10%) having mutations in POLR1C (Thiffault et al., 2015; Gauquelin et al., 2019) or POLR3K (Dorboz et al., 2018). This evidence concerns the gene POLR3K and leukodystrophy.