Figure 1: A complex multidirectional relationship between differentiated cancer cells and heterogeneous BCSCs. Depending on genetic and epigenetic changes, miRNA and gene regulation or microenvironment stimuli, cancer cells are able to shift between stem-like and non-stemlike states. This plasticity contributes to the heterogeneity of BCSCs. The regulatory mechanisms underlying the shift from non-BCSCs to BCSCs also play important roles in maintaining the BCSC pool and its characteristics. The currently used cell surface markers for BCSC identification could be potentially useful clinical biomarkers.