1. For reviews, see: J. Lykken, hep-ph/9511456; J. Lopez, Rept. Prog. Phys. 59 (1996) 819; K.R. Dienes, Phys. Reports 287 (1997) 447; Z. Kakushadze, hep-th/9707249; A. Faraggi, hep-ph/9707311; F. Quevedo, hep-ph/9707434; G.B. Cleaver, hep-th/9708023; Z. Kakushadze, G. Shiu, S.H.H. Tye, Y. Vtorov-Karevsky, hep-th/9710149.
2. I. Antoniadis, C.P. Bachas, C. Kounnas, P. Windey, Phys. Lett. B 171 (1986) 51; H. Kawai, D.C. Lewellen, S.-H.H. Tye, Phys. Rev. Lett. 57 (1986) 1832; I. Antoniadis, C.P. Bachas, C. Kounnas, Nucl. Phys. B 289 (1987) 87; I. Antoniadis, C.P. Bachas, Nucl. Phys. B 298 (1988) 586; H. Kawai, D.C. Lewellen, J.A. Schwarz, S.-H.H. Tye, Nucl. Phys. B 299 (1988) 431.
3. J. Ellis, S. Kelley, D.V. Nanopoulos, Phys. Lett. B 249 (1990) 441; B 260 (1991) 131; C. Giunti, C.W. Kim, U.W. Lee, Mod. Phys. Lett. A 6 (1991) 1745; U. Amaldi, W. de Boer, H. Furstenau, Phys. Lett. B 260 (1991) 447; P. Langacker, M.-X. Luo, Phys. Rev. D 44 (1991) 817.
4. P. Horava, E. Witten, Nucl. Phys. B 460 (1996) 506; E. Witten, Nucl. Phys. B 471 (1996) 135; P. Horava, E. Witten, Nucl. Phys. B 475 (1996) 94; see also: J. Schwarz, hep-th/9607201, and references therein.
5. See, for example, D.V. Nanopoulos, hep-th/9711080.