1. Dean, A. C. R., and Hinshelwood, C. N., Ciba Found. Symp., Drug Resistance in Micro-organisms, edit. by Wolstenholme and O'Connor, 4 (Churchill, London, 1957).
2. Albert, A., Rubbo, S. D., Goldacre, R. J., Davey, M. E., and Stone, J. D., Brit. J. Expt. Path., 26, 160 (1945).
3. Brenner, S., Barnett, L., Crick, F. H. C., and Orgel, A., J. Mol. Biol., 3, 121 (1961); Orgel, A., and Brenner, S., J. Mol. Biol., 3, 762 (1961).
4. Peacocke, A. R., and Skerrett, J. N. H., Trans. Faraday Soc., 52, 261 (1956).
5. Lerman, L. S., J. Mol. Biol., 3, 19 (1961).