RNAstructure is a Windows program for the prediction and analysis of RNA secondary structure. It works with Windows ME, Windows NT 4, Windows 2000, and Windows XP. Version 4.3 includes a secondary structure prediction algorithm, a sequence editor, an integrated drawing tool, the OligoWalk program, OligoScreen, Dynalign, and a partition function calculator. RNAstructure 4.3 uses the most current thermodynamic parameters from the Turner lab.