Differentiation of pluripotent embryonic stem (ES) cells into specific lineages is an important source of cells for implantation and gene delivery, as well as a useful model to study patterns of differentiation and gene expression during the very early development of the mammalian embryo (1 ). Embryonic stem cells are derived from the blastocyst inner cell mass (2 ,3 ), and remain totipotent when grown on the surface of embryonic fibroblasts or on gelatincoated substrates in the presence of leukemia inhibitory factor (LIF). ES cells appear to have unlimited proliferative capability, and, remarkably, when returned to the inner cell mass after culture and gene manipulation, resume their development and participate fully in the formation of ALL tissue types. Recently, embryonic stem cells have been derived from human blastocysts after in vitro fertilization (IVF) (4 ,5 ). Pluripotent stem cells have also been derived from human primordial germ cells (6 ), with obvious clinical applications.
