[1] Univ Alabama Birmingham, Dept Cell Dev & Integrat Biol, Birmingham, AL 35294 USA
[2] Birmingham Southern Coll, Dept Biol, Birmingham, AL 35254 USA
[3] Univ Oxford, Wellcome Trust Ctr Human Genet, Oxford OX3 7BN, England
[4] Univ Med Ctr Utrecht, Dept Cell Biol, NL-3584 CX Utrecht, Netherlands