Quiescent or slow-cycling stem cells in adult tissues can retain BrdU over long periods by either segregating chromosomes asymmetrically or dividing slowly. Label-retaining cells can be used to identify populations that contain stem cells [20] . In fact, many such studies have been used to determine putative stem cell locations in mammalian tissues [21, 22] . Using CCSP and BrdU double staining by IF, we found that 1.59% (39/2450) of cells in TBs and only 0.39% (12/4138) of them in BLs were BrdU + / CCSP + (Figure 6 ). The results suggest that the majority of mouse airway CCSP + stem/progenitor cells may reside in TBs.