van Kuilenburg ABP, et al. - In the 5′ untranslated region of the gene encoding glutaminase (GLS), an expansion of a GCA-repeat tract caused an inborn error of metabolism. This inborn error was identified through thorough clinical and biochemical phenotyping, in combination with whole-genome sequencing. Three unrelated patients who presented with an early-onset delay in overall development, progressive ataxia, and elevated levels of glutamine displayed this expansion. Cerebellar atrophy was reported in one patient in addition to ataxia. Researchers noted an association of the expansion with a relative deficiency of GLS messenger RNA transcribed from the expanded allele, which might have resulted from repeat-mediated chromatin changes upstream of the GLS repeat. The findings emphasize the significance of thorough examination of regions of the genome that are typically excluded from or poorly captured by exome sequencing.