RB1 and neoplasm: In addition, the effect of PPa extract on the Rb protein, a hallmark of the G1 to S transition during the cell cycle, and p21WAF1, which has been characterized as a tumor suppressor due to its role in mediating cell cycle arrest, has been examined (Jung et al., 2010; Giacinti & Giordano, 2006; Abbas & Dutta, 2009).