To conclude, we showed for the first time in an orthotopic GBM model, the U87-MG mouse model of glioblastoma and the suitability of a sig1R-targeting PET radioligand, (S)-(−)-[18F]fluspidine, to investigate the tumour-specific expression pattern of sig1R by in vivo imaging. The gene discussed is SIGMAR1; the disease is glioblastoma.