1. Applied Biosystems, Framingham, Massachusetts, USA
2. Laboratory of Cellular and Molecular Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, USA
3. Physical and Chemical Properties Division, National Institute of Standards and Technology, Gaithersburg, Maryland, USA
4. Unit on Biological Computation, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, USA