CDK1 and cancer: RNA level of the cancer stem cells markers (ALDH1A1 and Sox2) and the cell cycle marker CDK1 were upregulated about 1.7~3-fold for cells on feeders (Fig. 2D), and the CDK inhibitor p16 was downregulated about 2.5-fold for the cells cultured on 3T3 feeder layer compared to monolayer cells, whereas p21 was slightly down regulated.