The pre-pro-B cell growth-stimulating factor (PPBSF) is a self-assembling complex of Interleukin-7 (IL7 or IL-7) and a variant beta-chain of hepatocyte growth factor (HGFbeta) (Lai & Goldschneider 2001). This 55 kDa heterodimer, unlike monomeric IL7, selectively stimulates proliferation and differentiation of pre-pro-B cells in a long-term bone marrow culture system and up-regulates Interleukin-7 receptor alpha chain (IL7R) expression on pre-pro-B cell surface. It has been postulated that PPBSF is the active form of IL7 that normally induces IL7R-low pre-pro-B cells to proliferate and differentiate into IL7R-high pro-B cells, which then proliferate and differentiate into pre-B-cells on stimulation with monomeric IL7 (Wei et al. 2002).