From MIT's Technology Review, v. 112/no. 1, January 2009.
Cells clean up unneeded proteins by shipping them to a structure called the proteasome, which chops them up. If the proteasome can't do its job, the cell eventually dies. By targeting a specific component of the structure, Proteolix has developed a proteasome inhibitor that is particularly deadly to cancer cells. In the right dose, it kills cancer with little damage to healthy tissue. A variation on the molecule targets the proteasomes in immune cells (which differ from those in normal cells), disrupting biochemical pathways that cause autoimmune disorders.
Note: The immune related work must be particularly early in its development as I wasn't able to find any mention of it on their "news" page. http://www.proteolix.com/news.html