USP18 and leukodystrophy: Leukodystrophies related to bi-allelic loss-of-function mutations in CSF1R, TREM2, TYROBP, LRRC33/NRROS or USP18 can be categorized as primary microgliopathies (Oosterhof et al., 2019; Meuwissen et al., 2016; Schwabenland et al., 2019; Dong et al., 2020; Smith et al., 2020).