1. 1Division of Immunochemical Genetics, M.R.C. Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3UJ, U.K.
2. 2Division of Immunochemical Genetics, M.R.C. Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3UJ, U.K.
3. 3Division of Immunochemical Genetics, M.R.C. Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3UJ, U.K.