Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, USA. cokus@mcdb.ucla.edu|Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, USA. cokus@mcdb.ucla.edu|Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, USA. cokus@mcdb.ucla.edu|Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, USA. cokus@mcdb.ucla.edu|Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, USA. cokus@mcdb.ucla.edu