GRIN2B and epilepsy: The effectiveness of memantine on drug‐resistant early‐onset epilepsy in individuals with gain‐of‐function GRIN2 variants (GRIN2A, GRIN2B and GRIN2D) is mixed,1, 87, 88, 89 and previously, memantine treatment did not improve seizure frequency in four patients with GRIN2B gain‐of‐function mutations.1