Progenitor imbalance model. A normal phenotype likely requires a fine balance between different progenitor populations (DP and DN). In normal cells, a balance between proliferation and senescence interplays with a balance between these putative progenitor populations. This promotes regulated generation of differentiated cells. In aggressive tumours, increased proliferation and decreased senescence influences the equilibrium between different progenitor populations. This may alter the differentiated/undifferentiated cell balance, promoting basal-like phenotypes associated with tumour progression.