International Review of Cell and Molecular Biology V O L U M E T W O E I G H T Y INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY Series Editors GEOFFREY H BOURNE 1949–1988 JAMES F DANIELLI 1949–1984 KWANG W JEON 1967– MARTIN FRIEDLANDER 1984–1992 JONATHAN JARVIK 1993–1995 Editorial Advisory Board ISAIAH ARKIN KEITH LATHAM PETER L BEECH WALLACE F MARSHALL ROBERT A BLOODGOOD BRUCE D MCKEE DEAN BOK MICHAEL MELKONIAN KEITH BURRIDGE KEITH E MOSTOV.