1. Department of Molecular Genetics, Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, U.K.
2. Department of Visual Science, Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, U.K.
3. Department of Biochemistry, Institute of Cellular Signalling, University of Nijmegen, Adelbertusplein 1, 6525 EK Nijmegen, The Netherlands