Recently, differentiation pathway of plasma cells from germinal center B cells has been clarified in detail. Most of bone marrow plasma cells are considered to be derived from germinal center B cells. Early plasma cells are detected in the peripheral blood, and in the bone marrow, immature, intermediate and mature plasma cells are identified by expression of adhesion molecules such as VLA-5 and MPC-1. Myeloma cells are also subclassified into immature, intermediate and mature myeloma cells. Immature myeloma cells can respond to interleukin 6 (IL-6) to proliferate, and circulate in the peripheral blood and markedly expand in relapse. Therefore, immature myeloma cells are considered to be clonogenic cells for myeloma, so-called myeloma precursor cells.