ABCG2 and small cell lung carcinoma: Using the KEGG database, differential metabolites were annotated to identify metabolic pathway differences between groups A and B, C and D, and E and F. For the comparison between groups A and B, the top five differential metabolic pathways were related to Vitamin digestion and absorption, ABC transporters, purine metabolism, the Intestinal immune network for IgA production, and small cell lung cancer (Fig. 5A).