SIGMAR1 and myasthenia gravis: Encouraged by the in vitro and in vivo findings on the specific expression of sig1R in orthotopically grown U87-MG tumours and the sig1R-specific binding of (S)-(−)-[18F]fluspidine, we proceeded with PET studies with (S)-(−)-[18F]fluspidine performed under baseline conditions in nude mice-bearing orthotopic U87-MG (n = 3).