A measurement of tumour T normalised to the brain (ΔT) was able to distinguish between IDH1‐mutant and ‐wild‐type glioma, with IDH1‐mutant mice exhibiting an average ΔT of ∼29% and IDH1‐wild‐type mice having an average ΔT of only ∼3%. Here, IDH1 is linked to central nervous system cancer.