1. R. L. Johnson, L. V. Goodrich, M. P. Scott, Departments of Developmental Biology and Genetics, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305-5427, USA.
2. A. L. Rothman, J. Xie, J. W. Bare, J. M. Bonifas, A. G. Quinn, E. H. Epstein Jr., Department of Dermatology, San Francisco General Hospital, University of California, San Francisco, CA 94110, USA.
3. R. M. Myers and D. R. Cox, Department of Genetics, Stanford University School of Medicine, Stanford, CA 94305, USA.