DuPont is becoming more assertive about asking U.S. researchers to obtain licenses for permission to use the Harvard OncoMouse. The company--arguing that its OncoMouse patents cover any transgenic animal predisposed to cancer--is also asking institutions to enforce the agreements. Some have complied readily, according to DuPont, but others, including the Massachusetts Institute of Technology and the University of California, appear to be dragging their feet. And a few researchers and administrators are up in arms.