Cell cycle regulation - three words which conjure in the minds of those conducting research in this area a myriad of proteins and biochemical pathways. In this examination, an overview of the mammalian cell cycle is presented with emphasis on the function of the negative growth regulatory protein, the product of the retinoblastoma susceptibility gene, pRb. Since the activity of this protein itself is regulated by phosphorylation on serine and threonine residues, more elaborate discussions on the enzymes involved in placing the phosphates on, and taking them off, are provided. The focus here is on the activity of the members of the type 1 class of serine/threonine phosphatases. More specifically, the role of PP1 in regulating cell cycle progression by dephosphorylating pRb during mitosis, thereby activating the growth suppressing function of pRb, is presented. Suggested avenues for further investigation regarding the functional significance, and ultimately the effect on cell cycle progression, of the complex between pRb and the type 1 phosphatases are also discussed.