Fig. (1) Live MCTS expressing GFP reporter of luminal breast CSCs imaged in 3D. Maximal Projections of a CK5Pro-GFP-T47D spheroid treated with 100 nM progesterone (P4) over 72 h, resulting in global expansion of the CSC sub-population (top panel). RU486 (RU) 1 μM blocks global expansion of this CSC phenotype in CK5Pro-GFP-T47D spheroids (bottom panel). See supporting information for time lapse videos of P4 induced CSC expansion and inhibition using 1 μM isotretinoin.