AQP4-Ab serum titres did not differ markedly between acute relapses of myelitis (median, 1:1000; n = 13) and ON (median, 1:2500; n = 6), and were relatively high in the only patient with acute relapse of simultaneous optic neuritis and myelitis (1:5000). The gene discussed is AQP4; the disease is optic neuritis.