PPARA and nicotine dependence: KEGG pathway clustering analysis was performed on these 1,472 correlated DEGs, indicating that they were significantly enriched in eight functional pathways (bile secretion, neuroactive ligand-receptor interaction, cytokine-cytokine receptor interaction, arginine and proline metabolism, protein digestion and absorption, PPAR signaling pathway, nicotine addiction, Nitrogen metabolism) (Fig. 4A).