Abbreviations gLE Genetic leukoencephalopathy GWMD Genetic white matter disorder WMA White matter abnormality Genetic white matter disorders (GWMDs) are neurological diseases that affect the white matter of the central nervous system. In contrast, some GWMDs common in the UK, including Pelizaeus-Merzbacher disease, vanishing white matter disease, and Aicardi-Goutières syndrome,5 were absent in the Finnish cohort. Second, a variable disease course with variants in the disease-causing genes may have contributed to absent white matter findings. Not all GWMDs are linearly progressive, as demonstrated by episodic deterioration in mitochondrial disorders (e.g. mitochondrial DNA depletion syndrome 5).1 Defects in mitochondrial transfer RNA synthetases, such as I AARS2 i , are known to cause variable phenotypes, and improvement of MRI abnormalities may occur.1 Third, four patients were younger than 12 months at the time of the latest brain MRI.