||P.060701-1 left column:"During the past decade research on the mechanisms of radiation-induced DNA damage has expanded dramatically, with an increasing recognition that such damage must be understood at the nanoscale based upon the underlying fundamental atomic and molecular interactions [ref 1]. Such studies have required the preparation of dry DNA films as targets. In order to characterize interactions between DNA molecules in a film and incident particles it is therefore necessary to measure both the optical and dielectric properties of DNA [primary source 2] and, in particular, determine the film density. Measurements of the density of dry DNA films have previously been performed using a variety of methods, with the result strongly related to the method used (see Table I) [primary sources 2–5]. The most commonly used value of the density of calf thymus (CT) DNA and other genomes (including human) as well as plasmid DNA is reported to be approximately 1.7 g/cm 3 [primary sources 6, 8, 9, 11, refs 7, 10]. This value was measured using the buoyant density technique, which is commonly used to determine the unknown density of a component against the known density of a solvent. This technique resolves a sample into components of differing buoyant densities upon centrifugation. The term “buoyant density” refers to the fluid density, in which the sample particles manifest no tendency either to float or to sediment."