Case Study: Use of Low Copy Number DNA Evidence in Criminal Trials

July 8, 2020

By Jill K. Sanders, Esq.

The “CSI effect” can refer to jurors believing in the infallibility of DNA evidence in criminal trials. However, is all DNA evidence reliable? For example, in People v. Cadman Williams, the Court of Appeals reviewed the use of low copy number (LCN) DNA evidence. Here, we discuss the Court’s decision.


Case Background

In 2008, the victim and his brother were involved in a dispute with a group of teenagers in the Bronx. During the altercation, the defendant fired a gun. Unfortunately, two bullets hit the victim, who later died from his injuries.

Thereafter, the defendant fled New York. Several months later, he was arrested. After his arrest, the gun used to kill the victim was recovered from a wall cavity in the apartment of the defendant’s former girlfriend.

At trial, an eyewitness identified the defendant as the shooter, and video footage placed him at the scene of the altercation. Additionally, the defendant’s former girlfriend testified. She explained he forced her to hide the gun and had admitted “he had just shot somebody.” The prosecution also introduced low copy number (LCN) DNA evidence that indicated it was likely the defendant’s DNA that was found on the gun.

After trial, the defendant was convicted of manslaughter and a weapons charge. He was sentenced to 20 years in prison.


What is Low Copy Number DNA Evidence?

Low copy number (LCN) is a DNA profiling technique. It takes a small amount of DNA and copies it so that a larger sample profile can be created for testing. It can help solve cases where there is only a minute amount of DNA left by the perpetrator. Yet, there is criticism of an increased risk of contamination of samples in the laboratory.

At trial, the prosecution had an expert testify regarding this DNA evidence. DNA from two contributors was found on the gun, and standard DNA testing was unable to link the defendant to this DNA. The expert testified that LCN DNA testing and a statistical analysis conducted using a proprietary forensic statistical tool (FST) yielded the conclusion that one of the DNA contributors was the defendant.

The defendant objected to the use of LCN DNA evidence and the FST analysis. Alternatively, he sought a Frye hearing to determine whether the LCN and FST methods were generally accepted as reliable by the relevant scientific community.


Is Low Copy Number DNA Reliable?

The defendant argued that the use of LCN DNA testing was highly debated within the forensic community due to the unreliability of the profiles generated. In support, he submitted an affidavit from an expert and ten scholarly articles. Further, only one public lab in the country performed such testing. As to the FST analysis, the defendant argued it had been developed by the New York City Office of Chief Medical Examiner (OCME) and had not been validated or reviewed by anyone else in the scientific community.

In response, the prosecution submitted the OCME’s validating studies and certifications. They further noted that while OCME was the only government facility in the US issuing results of LCN DNA testing, there were many private labs and universities using the process. And the LCN procedure was not based on new or novel scientific techniques. Regarding the FST analysis, the prosecution claimed the software used mathematical and analytical tools that were generally accepted in the scientific community.



The Court concluded the defendant had raised sufficient questions regarding the general acceptance of the LCN DNA evidence. The prosecution was unable to cite any New York appellate cases, or out-of-state case law, assessing the general acceptance of this type of evidence. Similarly, the Court held the FST software had not been appropriately validated or reviewed by anyone else in the scientific community.

As such, the Court held “the trial court abused its discretion as a matter of law in admitting [the LCN DNA evidence and FST analysis] without holding … a [Frye] hearing.” However, the Court held this error was harmless given the other evidence of the defendant’s guilt. As such, the defendant’s conviction stands, and he continues to serve his sentence.