This paper by Arthur B. Pardee provides evidence that normal animal cells possess a unique regulatory mechanism, known as the restriction point (R-point), which allows them to shift between proliferative and quiescent states. The R-point is a critical point in the cell cycle, located in the G1 phase, where cells can be arrested under various suboptimal nutritional conditions. When these conditions are restored, cells can re-enter the S phase and resume proliferation. The study uses a variety of methods, including thymidine incorporation assays, to demonstrate that cells arrested by different means are indeed at the same point in the cell cycle. The R-point control is proposed to be a mechanism for normal cells to survive under conditions that would induce differentiation or nutritional deprivation. In contrast, malignant cells are suggested to have lost this control, leading to reduced survival under adverse conditions. The paper also discusses the implications of the R-point for understanding normal differentiation and the origin of cancer.This paper by Arthur B. Pardee provides evidence that normal animal cells possess a unique regulatory mechanism, known as the restriction point (R-point), which allows them to shift between proliferative and quiescent states. The R-point is a critical point in the cell cycle, located in the G1 phase, where cells can be arrested under various suboptimal nutritional conditions. When these conditions are restored, cells can re-enter the S phase and resume proliferation. The study uses a variety of methods, including thymidine incorporation assays, to demonstrate that cells arrested by different means are indeed at the same point in the cell cycle. The R-point control is proposed to be a mechanism for normal cells to survive under conditions that would induce differentiation or nutritional deprivation. In contrast, malignant cells are suggested to have lost this control, leading to reduced survival under adverse conditions. The paper also discusses the implications of the R-point for understanding normal differentiation and the origin of cancer.