In MM patients treated with CAR T cells against B cell maturation antigen (BCMA), 1 (8%) out of 12 treated patients exhibited biopsy-proven BCMA-loss at relapse, 8 patients remained BCMA-positive, and 3 were not evaluable for BCMA expression (no biopsy at relapse/progression) [21]. The gene discussed is TNFRSF17; the disease is Miyoshi myopathy.