RB1 and cancer: Various intracellular markers, such as Ki-67, c-Myc, Cyclin D, p27Kip1, or Rb, have been applied to detect the quiescent state of cancer cells, while most in vitro models of QCCs rely on alteration of tumor micro-environment, such as nutrient deprivation, hypoxia, contact inhibition, or anti-cancer treatment [7].