Current successes in targeted therapy, exemplified by the detection of anti-aquaporin-4 (AQP4) antibodies in neuromyelitis optica spectrum disorder (NMOSD), directly determine patient eligibility for treatments targeting complement (e.g., eculizumab) or the IL-6 receptor (e.g., satralizumab), facilitating a transition from “disease diagnosis” to “targeted therapy.” The gene discussed is AQP4; the disease is neuromyelitis optica.