The development of cancer is a multistep process. To understand oncogenesis and adapt appropriate treatments it is important to have a better definition of a number of factors, including the number and order of oncogenic steps, the identity of the targeted cells and deregulated cellular components, and the genes and pathways altered at each step. We propose here a hypothesis of oncogenesis based on the targeting of the cell cycle in two major steps. Oncogenic hits may occur in two sequences: in one scenario a first oncogenic hit alters the regulation of the G1 phase of the cell cycle leading to a proliferative, premalignant syndrome; oncogenesis is completed when a second oncogenic hit relieves the checkpoints of the late phases of the cell cycle. Alternatively, a genetic alteration may hit the late phases first; this leads to a premalignant disease with signs of senescence. In this scenario, the second hit targets the G1 phase. In the two sequences, oncogenesis is based on the cooperation of two hits targeting different phases of the cell cycle and relieving major checkpoints. Stem cells and progenitor cells of various tissues may be variably sensitive to these hits.

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