Abstract: The invention relates to a method of detecting an altered behaviour in a population of cells, said method comprising determining at least one of the following characteristics of the population of cells; (i) the proportion of stem cells, proliferating cells and differentiated cells in said cell population; or (ii) the size of stem cell clusters in said cell population; or (iii) the separation of stem cell clusters in said cell population; and comparing said at least one characteristic to a reference value, wherein a difference between the determined value and the reference value indicates an altered behaviour in said population of cells. Preferably the cells are mammalian, more preferably human epithelial cells, more preferably human epidermal cells.