[1] Heidelberg Univ, Zentrum Mol Biol, D-69120 Heidelberg, Germany
[2] Univ Geneva, Dept Mol Biol, CH-1211 Geneva 4, Switzerland
[3] Deutsch Krebsforschungszentrum, Abt Mol Biol Mitose, D-69120 Heidelberg, Germany
[4] Max Delbruck Ctr Mol Med, D-13122 Berlin, Germany