Editorials: CEll CyClE FEaturEs www.landesbioscience.com Cell Cycle 2149

Cell-to-cell heterogeneity in gene expression is intrinsic to human cancers. Recent work has shown that heterogeneity provides a set of molecular states allow-ing for flexible cell fate decisions within a group of cancer cells.1,2 For example, tumor-propagating cells adopt a prolifera-tive state to grow the overall population, whereas tumor-initiating cells can survive in harsh conditions and establish a new colony elsewhere in the body. For breast cancer, cells with tumorigenic potential are often identified by the expression of marker molecules, including CD44, CD133, α-6 integrin/CD49f, β-1 integ