1. Gupta, I., et al.: Naturwissenschaften 69, 453 (1982)
2. Wilkins, T.D.: private communication
3. Bruce, W.R:, et al.: Banbury Report 7, p. 227. Cold Spring Harbor 1981
4. Kingston, D.G.I., et al.: ibid., p. 215
5. Bruce, W.R., et al.: Conference on Cell Proliferation 3, p. 1641. Cold Spring Harbor 1977