Multi-level rule-based model of yeast cell cycle regulation (after ). (a) Nested view of the model structure. The world node contains a population of cells, each of which has two attributes, volume and cell cycle state. Stacking of boxes representing each entity indicates a variable number of instances. Each cell contains a population of molecules that comprise the biochemical components of the cell cycle. (b) Upward causation - components of the molecular layer influence dynamics of the cell layer. The number of MPF molecules in the I state (for 'inactive') controls the passage from G1 to S. The formula beneath the arrow must be true in order for the rule to fire. (c) Downward causation - the state of the cell influences the rate of a biochemical transformation of contained molecules.