Li-Fraumeni syndrome (LFS1) is a rare hereditary disorder that leads to a predisposition to cancer. This hereditary disorder is linked to mutations in the tumor suppressor gene encoding the transcription factor p53. p53 acts at the G1 checkpoint. If damaged DNA is detected, p53 halts the cell cycle. As p53 levels rise, the production of p21 is triggered. p21 enforces the halt in the cell cycle. A variant of Li-Fraumeni, called LFS2, is thought to occur due to a mutation of the CHK2 gene, which is also a tumor suppressor gene. CHK2 regulates the action of p53. Which of the following cascades is most likely to occur in a normal cell that does not contain the LFS mutation?