In this study, the expression of well documented stem cell markers such as CD44 [35] , CD133 [36, 37] , Sca1 [14, 38] and Sox2 [39, 40] was detectable by qRT-PCR in CCSP + cells. However, the level of CD133, Sca-1 and Sox2 expression was lower in CCSP + cells than that in CCSP 2 cells. One possible explanation is that CCSP is a Clara cell differentiation marker, so a CCSP + population of cells may contain more mature Clara cells, but few stem/progenitor cells, while CCSP 2 cells fraction is a mixture of many cells, such as type I, type II, ciliated cells, basal cells, smooth muscle cells and fibroblast cells and so on. Many of the cells have been shown to have stem cell features [19, 41] . Sphere culture showed that CCSP + cells were able to form spheroid colonies. The sphere colony size and efficiency of colony formation were lower in CCSP + cells compared to CCSP 2 cells. This data further suggests that CCSP + cells do have stem cell features, but stem cell activities are lower than in CCSP 2 cells. Using tissue sections, we found that 1.59% of CCSP positive cells in TBs were label-retaining cells and 1.1% were CCSP/SPC double positive BASCs. These data provide in vivo validation for the in vitro results that the CCSP + cell population contains a small subset of stem cells in the airway.