Here are two examples of external factors that influence the cell cycle:

1. Growth factors: These are proteins that stimulate cell division. They bind to receptors on the cell surface, triggering a cascade of signaling events that lead to the activation of genes involved in cell cycle progression. For example, platelet-derived growth factor (PDGF) promotes the growth and division of fibroblasts, which are cells that produce connective tissue.

2. Nutrients and oxygen availability: Cells require a constant supply of nutrients and oxygen to fuel their metabolic processes, including cell division. If these resources are scarce, cells will enter a state of quiescence (G0 phase) where they stop dividing. Conversely, an abundance of nutrients and oxygen will stimulate cell cycle progression. For example, cancer cells often exhibit uncontrolled growth due to their ability to thrive in nutrient-poor environments.