The majority of nondonor alleles matched the partner’s reference DNA profile, indicating that cohabitation affected nondonor DNA obtained from fingernail swabs. The findings showed that as the couples spent increasing amounts of time together, the incidence of mixed DNA profiles increased. The large variation in the incidence of foreign alleles observed within and between couples suggested that a combination of lifestyle factors had a significant effect on whether mixed DNA profiles were observed in the fingernail swabs. Further DNA profiling with the AmpFlSTR Y-filer polymerase chain reaction (PCR) amplification determined that 63 percent of the samples analyzed using Yfiler produced full or partial Y-single tandem repeat (STR) profiles. The Y-STR profiles were compared to partners’ profiles and searched through two haplotype databases in order to determine the evidential value of such samples. Although these results were from a small sample set and the use of either autonomic or Y-STR analysis on fingernail samples and the value of the results are dependent on the circumstances of the cases, these results provide important background information for forensic scientists. Fingernail swabs were taken from all fingers of the left and right hands of 12 couples on 3 separate occasions. DNA profiles were compared to both the donor and partner’s reference DNA profiles. 1 table, 7 figures, and 17 references