Phenotypic and genetic ITH measured from immunofluorescence staining and FISH (for commonCNAs) in 15 invasive breast tumours, containing bothin situ and invasive subregions within thesame tissue section. There was a high degree of intra-tumour variability in the expression ofmarkers for stem-like cells (CD44+) and more differentiated cells (CD24+). There was also a highdegree of genetic heterogeneity both within and between these distinct tumour cell populations. The gene discussed is CD44; the disease is neoplasm.