Analysis of multiple genetic loci by using short tandem repeats (STR) is widely used in human identity testing, because the extensive polymorphism at these loci allows for a high degree of discrimination among individuals. The authors recently received a forensic case that included several pieces of evidence and reference blood samples. Upon initial testing, one of the suspects had a DNA profile that included three alleles at four of the nine loci tested (vWA, FGA, TH01, and D5S818). At each locus, two of the alleles appeared to be "major" alleles with a third "minor" allele present. The profile appeared to be a mixture of two people. Contamination of this first reference sample was suspected; and a second, unopened blood specimen was requested from this individual. The DNA profile from this second reference specimen was identical to the original specimen at each locus. One of the evidence samples also displayed an identical mixed DNA profile that matched that of the reference specimens. The relative peak heights of the two "major" and one "minor" allele remained constant in all three samples. Additional background information showed that the suspect had not received a bone marrow transplant or blood transfusion; however, it was found that this individual is a fraternal twin. The researchers hypothesize that an exchange of blood cells between the fetuses occurred in utero and that the additional alleles present in these reference samples are derived from cells contributed by the suspect's twin sibling. No additional specimens from the suspect or his twin could be obtained for confirmation, and the hypothesis remains untested. Forensic scientists should be aware of this possibility when faced with a DNA profile in which extra alleles at multiple loci are detected. 1 table, 2 figures, and 24 references