MMP-12 is implicated in CS-induced emphysema (Belvisi and Bottomley 2003); however, MMP-12 was not elevated in SS-exposed mice (not shown), suggesting that, although classical emphysema is progressive and results from dissolution of preformed alveolar septae, the SS-induced BPD results from impaired formation of secondary alveolar septae and, in the absence of additional lung insults, is not progressive. This evidence concerns the gene MMP12 and pulmonary emphysema.