Three distinct luminal progenitors exist in the human mammary gland. (A) Left: distribution of CD49f and epithelial cell adhesion molecule (EpCAM) in the Lin- population from three patients. Right: distribution of aldehyde dehydrogenase (ALDH) and ERBB3 in the luminal progenitor subset. (B) Summary of distribution of luminal progenitor populations and the age of the patients. (C) Expression of keratin (KRT)8, MUC1, KRT5 and KRT14 among the three luminal progenitor populations. (D) Gene expression analysis of ALDH+, ALDH- and ERBB3- subpopulations relative to the comparator populations (nonclonogenic luminal (NCL)) for oestrogen receptor (ER), KRT14, MFG-E8, epidermal growth factor receptor (EGFR), GATA3, ERBB2, MUC1, ALDH1A3, LFT, AR, DACH1, ELF5, FOXA1, KRT19, myosin light chain kinase (MYLK), PR and snail homolog 2a (SNAI2A). (E) Frequency and distribution of colony-forming cells (CFCs) among the three luminal progenitor populations. (F) H & E and immunostained sections of xenograft gels derived from ALDH+, ALDH- and ERBB3- progenitors. Shown is the expression of MUC1, GATA3, SMA, p63, ER and prolactin-induced protein (PIP) among outgrowths. All error bars indicate the standard error of the mean from at least five independent samples. *P < 0.05, **P < 0.01, ***P < 0.0001. Scale bars = 10 μm.