Hypothetical model of the differentiation hierarchy within the mammary epithelium and potential relationships with breast cancer subtypes. NF-κB activation through the canonical or the non-canonical pathway is depicted as p50/p65 or p52/RelB heterodimers, respectively. Cell surface markers used for the isolation of human epithelial cell subsets are shown in blue. Adapted from [32, 33]. ER, oestrogen receptor; PR, progesterone receptor.