Nature 372, 150–157 (1994); doi:10.1038/372150a0 and correction Nature 380, 559 (1996); doi:10.1038/380559a0

We wish to retract this Article owing to an inability to reproduce the results. This retraction has not been signed by K.B. and A.F., and M.P. is deceased (J.O’B. cannot be traced).