This is version 4.2 of nab (nucleic acid builder), a computer language that is intended for use in creating, describing and modifying macromolecules, especially nucleic acids. Programs written in nab are translated into C, which in turn is compiled into machine code. Further general information can be obtained from our web page: WWW: http://www.scripps.edu/case Although we have been using this code for some time now, users should be aware that bugs may still exist. Please see the file COPYING for the usual disclaimer of liability. Further enhancements are planned, and will be announced on the web page. Please send comments and suggestions to Tom Macke (macke@scripps.edu) and/or to Dave Case (case@scripps.edu). If you find the program useful, please send us a note outlining your applications. This port to FreeBSD by M. L. Dodson, Univ. of Texas Medical Branch, Galveston bdodson@scms.utmb.edu