Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina, United States of America.|Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Mathematics, Duke University, Durham, North Carolina, United States of America.|Department of Mathematics, Duke University, Durham, North Carolina, United States of America.|Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina, United States of America.