[1] Univ Illinois, Dept Biochem & Mol Biol, Chicago, IL 60612 USA
[2] Univ Chicago, Howard Hughes Med Inst, Chicago, IL 60637 USA
[3] Burnham Inst, La Jolla, CA 92037 USA
[4] Kumamoto Univ, Sch Med, Inst Mol Embryol & Genet, Dept Cell Differentiat, Kumamoto 860, Japan