A subpopulation of CCSP + /pro-SPC + cells known as bronchioalveolar stem cells (BASCs) are capable of differentiating into Clara cells and alveolar type II cells and are considered to be adult lung stem cells [14] . In the current study, a rare portion of sorted CCSP + cells were also found to express the type II cell marker pro-SPC ( Figure 3D ). In order to confirm the existence of BASCs in vivo, we performed CCSP/pro-SPC double staining by IF in mouse lungs. Our results showed that 1.1% of TB epithelial cells contained BASCs while no CCSP + /pro-SPC + double positive epitheliums were detected in BLs ( Figure 6 ). In addition, a number of stem cell markers such as CD44, Sox2 and ALDH1 were detected by IF or IHC along the TB epithelium ( Figure 6C-E) . We also found that CD133, CD44, Sca-1 and Sox2 mRNAs were expressed at variable levels in mouse lung tissues ( Figure 6F, 6G) . This provides further evidence for the progenitor role that Clara cells may have in the mouse lung. 