In conclusion, the data obtained in the U87-MG mouse model of GBM along with the detection of sig1R in human GBM tissue for the first time by a PET radioligand, indicate not only the relevance of this target but also the suitability of (S)-(−)-[18F]fluspidine for sig1R-targeted cancer research and drug development. Here, SIGMAR1 is linked to cancer.