Analysis of single-nucleotide polymorphisms and their association with diseases and nondisease phenotypes is of growing importance in human biology studies. In this laboratory exercise, students determine the genetic basis for their ABO blood type; however, no blood is drawn. Students isolate genomic DNA from buccal mucosa cells that are present in saliva and analyze the DNA on an agarose gel. Subsequently, this DNA is used as a PCR template to amplify exons 6 and 7 of the gene that determines the human ABO phenotype. These PCR products are digested and run on agarose gels to examine the restriction fragment length polymorphisms. A deletion in the O[superscript 1] allele converts the BstE II site in exon 6 into a "Kpn" I site, and this feature is used to determine the presence of O[superscript 1] alleles. The pattern of exon 7 digest products allows students to distinguish among four other common ABO alleles: A[superscript 1], A[superscript 2], B, and O[superscript 2]. This exercise introduces students to commonly used molecular biology techniques, such as genomic DNA isolation, PCR, gel electrophoresis, and restriction digests. (Contains 1 footnote, 5 figures and 2 tables.)