This movie is a confocal Z-stack of eight-week-old Fsp-Cre; Ptch1fl/fl; mTmG+ mammary ducts showing some ducts with clear lumens, and a duct filled with RFP+ cells (upper right). GFP and RFP are endogenous fluorescence from the mTmG reporter; GFP indicates Fsp-Cre+ cells, while RFP is expressed by non-Fsp+ cells.