Figure 1.

Methodology for gene classification by degree of reversibility upon smoking cessation. For each probeset, the relationship between gene expression in log₂ scale (ge), age, current smoking status (x_curr), former smoking status (x_form), and the interaction between former smoking status and months elapsed since quitting smoking (x_tq) was examined with the linear regression model. Genes differentially expressed between current (C) and never (N) smokers were categorized based on their behavior in former smokers (F) relative to never smokers as a function of time since smoking cessation. Genes were classified as 'rapidly reversible' if there was not a significant difference between former and never smokers. Genes were classified as 'indeterminate' if there was a significant difference between former and never smokers, but the age-adjusted fold change between former and never smokers was not greater than or equal to 1.5. If the fold change criterion was met, genes were classified as 'slowly reversible' if there was a significant relationship between gene expression and time since quitting smoking or as 'irreversible' if there was not a significant relationship with time.