1. 1. Evans, M. J. & Kaufman, M. H. Nature 292, 154–156 (1981). 2. Gossler, A., Doetschman, T., Korn, R., Serfling, E. & Kemler, R. Proc. natn. Acad. Sci.U.S.A. 83, 9065–9069 (1986). 3. Robertson, E., Bradley, A., Kuehn, M. & Evans, M. Nature 323, 445–448 (1986). 4. Smithies, O., Gregg, R. G., Boggs, S. S., Koralewski, M. A. & Kucherlapati, R. S. Nature 317, 230–234 (1985). 5. Hooper, M., Hardy, K., Handyside, A., Hunter, S. & Monk, M. Nature 326, 292–295 (1987). 6. Kuehn, M. R., Bradley, M., Robertson, E. J. & Evans, M. J. Nature 326, 295–298 (1987). 7. Smith, A. J. H. & Berg, P. Cold Spring Harb. Symp. quant. Biol. 149, 171–181 (1984). 8. Smithies, O., Koralewski, M. A., Song, K.-Y. & Kucherlapati, R. S. Cold Spring Harb. Symp. quant. Biol. 49, 161–170 (1984). 9. Lin, F.-L., Sperle, K. K. & Sternberg, N. Cold Spring Harb. Symp. quant. Biol. 49, 139–149 (1984). 10.Lin, F.-L., Sperle, K. & Sternberg, N. Proc. natn. Acad. Sci. U.S.A. 82, 1391–1395 (1985). 11.Rubnitz, J. & Subramani, S. Molec. cell. Biol. 6, 1608–1614 (1986). 12.Thomas, K. R., Folger, K. R. & Capecchi, M. R. Cell 44, 419–428 (1986). 13.Gregg, R. G. & Smithies, O. Cold Spring Harbor Symp. quant. Biol. 51, 1093–1099 (1986). 14.Song, K.-Y., Schwartz, F., Maeda, N., Smithies, O. & Kucherlapati, R. S. Proc. natn. Acad. Sci. U.S.A. 84, 6820–6824 (1987). 15.Boggs, S. S., Gregg, R. G., Borenstein, N. & Smithies, O. Expl Hemat. 14, 988–994 (1986). 16.Smith, A. G. & Hooper, M. L. Devl Biol. 121, 1–9 (1987). 17.Southern, P. J. & Berg, P. J. Molec. Appl. Genet. 1, 327–341 (1982). 18.Melton, D. W., Konecki, D. S., Brennand, J. & Caskey, C. T. Proc. natn. Acad. Sci. U.S.A. 81,2147–2151 (1984). 19.Ayares, D., Ganea, D., Chekuri, L., Campbell, C. R. & Kucherlapati, R. Molec. cell. Biol. 7, 1656–1662 (1987).