Expression profiles in airway epithelia of current (C), former (F), and never (N) smokers. Top: colormap of pathway activity levels for the highest ranking pathways in the comparison between current smokers and never smokers. Bottom: colormap for genes in the KEGG glutathione metabolism pathway. Glutathione is an important anti-oxidant known to be increased in the lungs of smokers. The genes with the highest weights in this pathway, GCLM and GCLC, encode the subunits of glutamate cysteine ligase (GCL), the rate-limiting enzyme in the synthesis of glutathione .