Taylar Hammond1,2 and Julien Sage1,3 1Department of Pediatrics, Stanford University, Stanford, California 94305, USA 2Department of Biology, Stanford University, Stanford, California 94305, USA 3Department of Genetics, Stanford University, Stanford, California 94305, USA Correspondence: julsagestanford.edu

Cell division is obligatory to tumor growth. However, both cancer cells and noncancer cells in tumors can be found in distinct stages of the cell cycle, which may inform the growth potential of these tumors, their propensity to metastasize, and their response to therapy. Hence, it is of utmost importance to monitor the cell cycle of tumor cells. Here we discuss well-established methods and new genetic advances to track the cell cycle of tumor cells in mouse models of human cancer. We also review recent genetic studies investigating the role of the cell-cycle machinery in the growth of tumors in vivo, with a focus on the machinery regulating the G1/S transition of the cell cycle.