Novel genetic variants of the Cytochrome P450, Subfamily IIIA, Polypeptide 5 (CYP3A5) gene are described. Various genotypes, haplotypes, and haplotype pairs that exist in the general United States population are disclosed for the CYP3A5 gene. Compositions and methods for haplotyping and/or genotyping the CYP3A5 gene in an individual are also disclosed. Polynucleotides defined by the haplotypes disclosed herein are also described.