We and others have previously reported loss of heterozygosity at the IDH1 locus in gliomas and leukemias (Ichimura et al., 2009; Zhang et al., 2011; Gupta et al., 2012), and monoallelic expression of IDH1 in gliomas is not uncommon (Walker et al., 2012). Here, IDH1 is linked to central nervous system cancer.