GGT1 and central nervous system cancer: Considering previous work showing that the ratio of [1-13C]glycine to total HP signal (γ-glutamyl-[1-13C]glycine plus [1-13C]glycine) is positively correlated with GGT activity31, previous studies showing higher levels of GGT in U87 glioma cells compare to normal brain19, and our observation that higher HP [1-13C]glycine production in tumor was associated with higher GGT expression in tumor tissue, our results collectively point to the potential of γ-glutamyl-[1-13C]glycine as a HP probe to detect brain tumor in vivo in real time.