Chronic smoke exposure induces selective differential gene expression profiles in wild-type (WT) versus Mmp28−/− mice. A: By directly comparing WT and Mmp28−/− mice exposed to cigarette smoke, we identified a limited set of genes that are up-regulated in WT relative to Mmp28−/− and mapped to neutrophil chemotaxis, immune and defense response, and cytokine activity. B: Box-and-whisker plot showing increased expression levels of four representative genes (Il1b, Cd14, Csf3, Cxcl2) in smoke-exposed WT mice.