Final answer:
The statement is false because proto-oncogenes stimulate the cell cycle, while tumor suppressor genes inhibit it.
Step-by-step explanation:
The statement that stimulating proteins are encoded by tumor suppressor genes and inhibitory proteins are encoded by proto-oncogenes is false. The correct roles of these genes are the opposite: proto-oncogenes code for proteins that stimulate the cell cycle and promote cell division, acting like the cell's own accelerator.
On the other hand, tumor suppressor genes produce proteins that inhibit the cell cycle and can induce cell death if necessary, functioning as the cell's brakes. For example, proteins such as p53, p21, and Rb are products of tumor suppressor genes and work to halt the progression of the cell cycle if problems are detected, while proto-oncogenes encode for proteins like growth factor receptors that push the cell cycle forward.