Recently, Wong and colleagues [13] reported the isolation of progenitor epithelial cells from mouse and human bone marrow. These cells expressed Clara cell secretory protein (Ccsp), a marker of airway progenitor cells [14] , CD45 and mesenchymal markers CD73, CD90, CD105. These cells differentiated into multiple epithelial cell lineages, including type I and II pneumocytes in vitro. Furthermore, these progenitor epithelial cells preferentially homed to naphthalene-injured lung following intratracheal or systemic inoculation.