The incidence of serious (⩾grade 3) NAEs is below 1%, and this encompasses a highly heterogeneous spectrum of neurological symptoms.57,71,73,74 Even if any-grade NAEs are more frequent with PD-1/PD-L1 antibodies (6.1%) than with anti-CTLA4 (3.8%), anti-CTLA4 seem to confer a higher risk for serious (grade 3–4) NAEs (0.7%) than anti-PD-1/PD-L1 antibodies (0.4%).71 Aseptic meningitis or encephalitis develops in 0.1–0.2% of patients on ICIs.23 This evidence concerns the gene CTLA4 and viral encephalitis.