UNLABELLED: The DNA inter base pair step parameters (Tilt, Roll, Twist, Shift, Slide, Rise) are a standard internal coordinate representation of DNA. In the absence of bend and shear, it is relatively easy to mentally visualize how Twist and Rise generate the familiar double helix. More complex structures do not readily yield to such intuition. For this reason, we developed a plug-in for VMD that accepts a set of mathematical expressions as input and generates a coarse-grained model of DNA as output. This feature of VDNA appears to provide a unique approach to DNA modeling. Predefined expressions include: linear, sheared, bent and circular DNA, and models of the nucleosome superhelix, chromatin, thermal motion and nucleosome unwrapping. AVAILABILITY: VDNA is pre-installed in VMD, http://www.ks.uiuc.edu/Research/vmd. Updates are at http://dna.ccs.tulane.edu.