CD27 and plasma cell myeloma: Previous controversial studies have identified myeloma CSCs in (i) the CD138-negative fraction, either among B cells (CD138 −/CD19 +)73,74 or memory B cells (CD19 +/CD27 +/CD20 +)75; (ii) the dominant malignant CD138-positive plasma cells76,77 or (iii) in the bidirectional transition from pre-plasma cells (CD19-/CD138-) to plasma cells (CD19 −/CD138 +), with clonogenic, tumorigenic and propagating potential11.