Clara cells have several protective properties. They detoxify xenobiotics and oxidant gasses, control inflammation, participate in mucociliary clearance of environmental agents, and proliferate/ differentiate to maintain the ciliated and non-ciliated cell population. Clara cells are a source of cytochrome P450 enzymes that contribute to the metabolism of a variety of substances [4] . In addition to the major Clara cell secretory protein (CCSP), also known as CC10, CC16, Clara cell antigen, secretoglobin 1A1 (SCGB1A1) or uteroglobin, Clara cells also contribute surfactant apoproteins A, B and D, proteases, anti-microbial peptides, several cytokines and chemokines, and mucins in the extracellular fluid lining airspaces. CCSP is the most abundant secretory protein found in the airway surface fluid, expressed exclusively in nonciliated Clara cells and widely used as a marker of the cells [5, 6, 7, 8] .Changes in CCSP levels have a profound impact on not only the composition of airway surface fluid but also the airway epithelial response to environmental stimuli [9, 10] . Another important property of Clara cells is their ability to serve as progenitors for airway lining cells in response to injury. Moreover, subpopulations of CCSP-expressing cells may function as true stem cells of adult airways. Presently it is not known whether the groups overlap or represent distinct cells such as variant Clara cells [11] , type A cells [12] , OCT4-expressing stem cells [13] and bronchioalveolar stem cells (BASCs) [14] .