Dear Ben, We definitely agree that molecule-building would be a very useful addition to Chimera. Toward this functionality, Chimera includes hydrogen addition; also, the "bond" command has been implemented (and will be available in the next release). Chimera also includes the "swapaa" and "swapna" commands which "mutate" one sidechain or base to another, although they are not very sophisticated. I am not certain what you are referring to in MIDAS, which only had very rudimentary building capabilities and no ability to add hydrogens. The "addgrp" command did allow definition of new amino acid sidechains, but was quite unwieldy and not usable for building in general. Similarly, "addaa" could append one of the standard amino acids to the end of a peptide but was not generalizable to other types of molecules. If you could clarify how you were applying these tools, we might be able to prioritize developments accordingly. Thank you, Elaine =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Elaine C. Meng, Ph.D. meng@cgl.ucsf.edu Computer Graphics Lab and Babbitt Lab Department of Pharmaceutical Chemistry University of California, San Francisco http://www.cgl.ucsf.edu/home/meng/index.html =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=