[1] Univ Leicester, Dept Biochem, Leicester LE1 7RH, Leics, England
[2] Univ Texas, Dept Microbiol, Inst Mol & Cellular Biol, Austin, TX 78712 USA
[3] Univ London Queen Mary & Westfield Coll, St Bartholomews & Royal London Sch Med & Dent, London EC1A 7BE, England
[4] Western Gen Hosp, MRC, Human Genet Unit, Edinburgh EH4 2XU, Midlothian, Scotland