United States v. McCluskey

Laboratories that conduct DNA testing may specify a level of DNA in a sample below which testing of the sample could produce unreliable results. This level of DNA is known as the stochastic threshold, and a quantity of DNA that falls below the threshold is classified as a low copy number (LCN). John Charles McCluskey, Tracy Province, and other individuals (defendants) were charged with murdering a couple near a highway rest stop in New Mexico. The United States (plaintiff) gathered swab samples of DNA from the victims’ car and a handgun allegedly used in the crime. One swab taken from the handgun’s magazine contained only 215 picograms of DNA, which was below the 250-picogram stochastic threshold set by the New Mexico Department of Public Safety (NMDPS). The government’s DNA analyst at the NMDPS laboratory, Carrie Zais Davis, conducted LCN testing on the sample from the magazine. Davis asserted that the testing showed to “a reasonable degree of scientific certainty” that Province was the sole source of DNA on the sample. Province and the other defendants moved to exclude Davis’s testimony, challenging the reliability of LCN DNA testing. At an evidentiary hearing, the district court asked Davis to justify her conclusion that the test results from the sample were reliable even though the amount of DNA was below the stochastic threshold. Davis responded simply that she believed based on her experience that the results were reliable, but she did not cite any scientific studies or other support for her assertion. Although the government cited some scientific support for the reliability of LCN testing results, the government’s authorities all involved different testing procedures from those used by the NMDPS laboratory.