S-2 Gastrointestinal Drug Absorption: Transporters Are Working   S2-1. Analysis for the Involvement of Drug Transporters in Gastrointestinal Absorption   S2-2. Efflux Transporter: Role of P-Glycoprotein (P-gp) on Intestinal Absorption and Species Differences of Its Substrate Recognition   S2-3. Interplay between Breast Cancer Resistance Protein (BCRP) and Metabolizing Enzymes in the Intestine   S2-4. Utilization of Intestinal Influx Transporters for Improvement of Drug Absorption   S2-5. Drug Absorption Enhancement Based on Efflux Transporter Inhibition   S2-6. Contribution of Intestinal Transporters to Drug Absorption: Strategy for Drug Discovery