1. Beaugé, L. A. & Glynn, I. M. Nature 268, 355–356 (1977).
2. Fagan, J. B. & Racker, E. Biochemistry 16, 158 (1977).
3. Josephson, L. & Cantley, L. C. Biochemistry 16, 4572–4578 (1977).
4. Cantley, L. C. et al. J. biol. Chem. 252, (in the press).
5. Hopkins, L. L. in Trace Element Metabolism in Animals Vol. 2. (eds Hoekstra, W. G., Suttie, J. W., Ganther, H. E. & Mertz, W.) 397–406 (University Park Press, Baltimore, 1974).