We think that Singulair requires help from a transport protein to be absorbed and that genetic variability in this transporter leads to variability in the blood level of Singulair. In this proposal we will use citrus juice (grapefruit and orange) to inhibit intestinal membrane transport proteins and show that Singulair requires these transporters to be efficiently absorbed. Eventually, what we learn from this work will allow doctors to quickly test individuals with asthma to determine how well they will absorb Singulair and possibly other LTRAs. Knowing this will allow the doctor to adjust the drug treatment on an individual basis to maximize benefit in the treatment of asthma.