This movie is a confocal Z-stack from an eight-week-old Fsp-Cre;Ptch1+/+;mTmG+ mammary duct showing a clear ductal lumen. GFP and RFP are endogenous fluorescence from the mTmG reporter; GFP indicates Fsp-Cre+ stromal cells, while RFP is expressed by Fsp- cells.