In need for a better understanding of the missing 18F-FET uptake in 18F-FET-negative gliomas, we evaluated the LAT1 expression levels in a homogeneous group of newly diagnosed IDH-mutant astrocytomas (without 1p/19q codeletion) and compared them with a neuropathologically and sex-matched 18F-FET-positive glioma group and correlated in vivo and in vitro parameters with clinical survival data. The gene discussed is SLC7A5; the disease is glioma.