For convenience the term quiescent cells will be used in this chapter to define normal or tumor cells, untreated by any specific drugs, that do not progress through the mitotic cycle for an extended period of time. The definition is purposefully wide to cover a variety of situations, often with different metabolic causes, associated with irreversible or transient cell withdrawal from the cell cycle. The even wider term, noncycling cells, will be reserved to describe, in addition to quiescent cells, cells arrested in the cycle by drugs (Table 1 ). (SeeChapter 3 for additional discussion of cellular quiescence.)
