The cell cycle is controlled by the products of two classes of genes, proto-oncogenes and tumor suppressor genes. Normally the expression of these genes is balanced. 1. Proto-oncogenes encode proteins that promote cell division (These proteins act like the gas pedal on a car.. Cyclins: a family of proteins that regulate the timing of the cell cycle. These proteins trigger cell division. Internal regulators: several proteins make sure the cell cycle does not proceed unless certain processes have happened inside the cell.