Cell cycle checkpoints are an important feature of a dividing cell to reduce errors that can occur in various stages of the cell cycle. Loss of Rb, an important part of the G1-S transition checkpoint, can result in uncontrolled cell cycle progression and cancer. All of the following would mimic loss of Rb except:

• Constitutively active G1-CDK
• Constitutively active ERK
• Constitutively active E2F
• Constitutively active Myc
• Constitutively active Ras GTPase activating protein