1. N. B. Cole, N. Sciaky, J. Lippincott-Schwartz, Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, Building 18T, National Institutes of Health, Bethesda, MD 20892, USA.
2. C. L. Smith, National Institute of Neurological Disorders and Stroke, Building 36, National Institutes of Health, Bethesda, MD 20892, USA.
3. M. Terasaki, Department of Physiology, University of Connecticut Health Center, Farmington, CT 06032, USA.
4. M. Edidin, Department of Biology, The Johns Hopkins University, Baltimore, MD 21218, USA.