The antiapoptotic proteins Bcl-2 and Bcl-xL may have nonhomologous func tions in hematopoietic development. A reciprocal pattern of expression for these proteins has been described in developing T and В cells, and subsets of adult human and murine bone-marrow cells (Gratiot-Deans et al.y 1994; Merino et al.y 1994; Park et al., 1995; Grillot et a l , 1996). BH proteins are linked to cell differen tiation decisions independent of survival activity. Bcl-2 induces differentiation in human neural-crest derived tumor cells (Zhang et al.y 1996) and rat pheochromocytoma PC12 cells (Sato et al., 1994). In a comparison of Bcl-xL and Bcl-2 in staurosporine-treated neuroblastoma cells, Bcl-xL, but not Bcl-2, was associated with robust neuronal differentiation of surviving cells (Yuste et al.y 2002).