The concept that solid tumors can be comprised of both actively cycling and noncycling cell populations was originally proposed by Mendelsohn (32 ) to account for the observation that after [3 H]-TdR (tritiated thymidine) labeling in vivo, the fraction of labeled mitotic cells always exceeded that for interphase cells. Since the mitotic population can be considered a pure cycling cohort, it was concluded that a significant portion of the interphase population must be in a noncycling or quiescent (Q) configuration. The term “growth fraction (GF)” was thus coined to distinguish the cell population, in the tumor, that is actively engaged in cell cycle traverse (P cells) from that that is not (Q cells).