The Ninth Annual Pezcoller Symposium entitled "The Biology of Tumors" was held in Rovereto, Italy, June 4-7, 1997. It focused on the genetic mechanisms underlying het- erogeneity of tumor cell populations and tumor cell differentiation, on interactions be- tween tumor cells and cells of host defenses, and the mechanisms of angiogenesis. With presentations at the cutting edge of progress and stimulating discussions, this symposium addressed issues related to phenomena concerned with cell regulation and cell interactions as determined by activated genes through the appropriate and timely media-...