The Diagnosis and Treatment Guidelines for Optic Neuromyelitis Spectrum Disorders recommend monitoring peripheral blood B lymphocyte subsets in patients with NMO receiving RTX treatment: When the proportion of CD19+ or CD20+ B cells is >1% or the proportion of CD27+ memory B lymphocytes is >0.05%, repeating RTX treatment is recommended (16). The gene discussed is CD27; the disease is neuromyelitis optica.