Classification is based exclusively on the functional ability of various cytokines to promote growth of primitive murine hematopoietic progenitor cells and candidate stem cells in vitro. In principle, this classification holds true for primitive human progenitor cells as well. Recruitment of primitive hematopoietic progenitor/stem cells generally can only occur through combined activation of at least two cytokine receptors, whereas optimal growth usually requires the synergistic interaction between multiple cytokines.2,4 Although usually having little or no growth-promoting activity alone, FL and/or KL can, with few exceptions, synergistically interact with any of the colony-stimulating factors and/or purely synergistic cytokines to enhance growth. Although synergy can occur between cytokines within one class, the most efficient growth-stimulation is obtained by combining cytokines from different classes.2 4