Yup, ... sure nuff, .... it's the cigarette smoke.

One of the deadliest aspects of a cancerous tumor is its ability to grow and spread, assimilating and destroying healthy cells throughout the body.

...... the protein E2F sometimes boosts tumor cell growth and at other times suppresses it.

According to La Thangue, when E2F is functioning properly, it is a crucial tool for cell regulation.  

.....  the protein helps healthy cells grow, but when a cell becomes damaged or sick, the protein reverts to Dr. Jekyll and signals for the cell to die. This process is called apoptosis, and it helps to prevent the buildup of DNA errors or the development of cancer.