Myeloma is characterized by an accumulation of malignant plasma cells in bone marrow. Numerous observations indicated that malignant plasma cells have low proliferative capacity and it is believed that the vast majority of the malignant cell population in myeloma is represented by terminally differentiated plasma cells, similar to their normal counterparts (Barlogie et al., 1989). It is unclear whether these terminally differentiated malignant plasma cells are capable of self-renewal. During the past decade, several studies indicated that multiple myeloma, similarly to other malignancies, may consist of heterogenous population of malignant cells (Bakkus et al., 1994; Billadeau et al., 1996).