Purification of HR+ cells by FACS in early pregnancy. (A) The luminal population of mammary epithelial cells (CD24hiCD49flo) was separated into hormone-sensing cells (Sca1hi CD49blo; purple gate) and alveolar progenitor cells (Sca1lo CD49bhi; orange gate). (B) Quantitative RT-PCR on 500 directly-lysed cells per sample for markers that distinguish the two luminal cell types. Values are relative to mRNA levels in HR+ cells from virgin animals and normalised to HPRT expression. Error bars indicate standard deviation for 3 individual mice.