It is thought that selection determines which cells in a tissue give rise to a tumour through clonal expansion. Identifying biomarkers that detect this early precancerous process might enable prevention before tumours form. Based on findings that Flower protein marks pretumoral cells in Drosophila, Petrova et al. set out to study whether the mouse homologue (mFwe) has the same function. They show that mFwe expression is increased in skin cells surrounding papillomas in mice, and that mFwe-deficient mice develop normally but are resistant to chemically induced skin papillomas, suggesting that mFwe promotes the expansion of pretumoral cells. These data suggest that Flower protein might be a biomarker for early skin cancer detection and a potential drug target. Page 553