Medical Research Council, Elsie Widdowson Laboratory, 120 Fulbourn Road, Cambridge, CB1 9NL, UK.|Medical Research Council, Elsie Widdowson Laboratory, 120 Fulbourn Road, Cambridge, CB1 9NL, UK.|Medical Research Council, Elsie Widdowson Laboratory, 120 Fulbourn Road, Cambridge, CB1 9NL, UK.|Medical Research Council, Elsie Widdowson Laboratory, 120 Fulbourn Road, Cambridge, CB1 9NL, UK.|Medical Research Council, Elsie Widdowson Laboratory, 120 Fulbourn Road, Cambridge, CB1 9NL, UK. Jules.Griffin@mrc-hnr.cam.ac.uk.