Figure 3: Suggested hierarchial order of breast cancer cell types (cell of origin) defined by expression analysis in relation to hormone receptor status. In the figure also is hypothesized where tumours developing in germline mutations carriers of p53, BRCA1, BRCA2 and BRCAX preferentially have its cell of origin. The hypothesis postulates that tumours originating in germline mutation carriers of BRCA1 mainly are ER-, in BRCA2 carriers mixed ER+ and ER- and in BRCAX carriers mainly ER+. The origin of the BRCA1 tumors recently has been associated with a luminal progenitor cell instead of a basal located stem cell (19) although by gene expression studies BRCA1 associated tumors display a basal phenotype. The cell of origin is also related to age at diagnosis with receptor positive tumours initiated at a higher age in a more terminally differentiated tissue.