1. 1. Lewis, E.B. Nature 276, 565–570 (1978). 2. Kaufman, T.C. & Abbott, M.K. in Molecular Aspects of Earlv Development (edsMalacinski, G.M. & Klein, W.H.) 189–218 (Plenum, New York, 1984). 3. Beeman, R. Nature327, 247–249(1987). 4. Gehring, W.J. in ICN-UCLA Svmposia on Molecular and Cellular Biology (1986). 5. Regulski, M. McGinnis, N., Chadwick, R. & McGinnis, W. EM BO J. 6, 767–777 (1987). 6. Sanchez-Herrero, E., Vernos, I., Marco, R. & Morata, G. Mem'313, 108–113(1986). 7. Struhl, G. Proc. natn, Acad. Sci. U.S.A. 79, 7380–7384 (1982). 8. Handing, K., Wedeen, C., McGinnis, W. & Levine, M. Science 229, 1236– 1242 (1985). 9. Akam, M. Development (in the press). 10. Akam, M. & Martinez-Ariaz, A. EMBOJ. 4, 1689–1700 (1985). 11. Chadwick, R. & McGinnis, W. EM BO J. 6, 779–789 (1987).