1. Department of Biochemistry, Carver College of Medicine; University of Iowa; Iowa City IA 52242 USA
2. Current address: Department of Neurology; Washington University School of Medicine; St. Louis MO 63110 USA
3. Department of Ophthalmology; West Virginia University School of Medicine and West Virginia University Eye Institute; Morgantown WV 26506 USA
4. Current address: Section for Translational Research in Retina & Macular Degeneration; National Institute on Deafness and Other Communication Disorders, NIH; Bethesda MD 20892 USA