STRmix™ Celebrates 10th Anniversary

Sophisticated Forensic Software Now Established in Forensic Labs Worldwide

On August 27, STRmix™ will celebrate 10 years since it was first introduced to the world as sophisticated forensic software, capable of resolving mixed DNA profiles which previously were regarded as too complex to interpret.

Since that time, STRmix™ has become the accepted standard for DNA mixture interpretation, used by forensic labs worldwide to analyze DNA evidence in more than 380,000 cases. That includes more than 210,000 cases in North America, 30,000 in Europe and the United Kingdom, and 135,000 in Australasia.

Today, STRmix™ is in use in all nine state and territory forensic laboratories in New Zealand and Australia; 16 forensic labs throughout Europe, the United Kingdom, Asia, the Middle East, Canada, and the Caribbean; and 72 forensic labs in the United States, including those operated by the FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

John Buckleton DSc, FRSNZ, Principal Scientist at the New Zealand Institute of Environmental Science and Research (ESR) and one of the developers of STRmix™, credits the software’s popularity to its highly successful track record in resolving low-level, degraded, or mixed DNA samples from multiple contributors.

According to Dr. Buckleton, “STRmix™ has proven to be very effective in producing usable, interpretable, and legally admissible DNA evidence in a wide range of criminal cases worldwide. It is particularly effective in resolving violent crime and sexual assault cases, as well as cold cases in which evidence originally dismissed as inconclusive was able to be reexamined.”

Unlike previous methods of DNA analysis, which depended on the application of fixed stochastic thresholds and other biological parameters to manually analyze DNA samples, “STRmix™ assesses how closely multitudes of proposed DNA profiles resemble or can explain an observed DNA mixture,” explains Dr. Jo-Anne Bright, Senior Science Leader at ESR, who developed STRmix™ with Dr. Buckleton and Dr. Duncan A. Taylor of Australia’s Forensic Science SA (FSSA).

Dr. Bright notes that STRmix™ relies on proven methodologies routinely used in computational biology, physics, engineering, and weather prediction “to calculate the probability of observed DNA evidence by assuming the DNA originated from either a person of interest or an unknown donor. These two probabilities are then presented as a likelihood ratio (LR), inferring the value of the findings and level of support for one proposition over the other.”

Dr. Taylor points out that incremental but significant changes have been made to STRmix™ since its introduction, usually in response to recommendations made by the forensic laboratories using the software. As a result, “The functionality and power within STRmix™ has grown significantly over the past 10 years. STRmix™ has revealed to us how much information is present in DNA profiles, and unlocked what was inaccessible for so long,” says Dr. Taylor.

The latest of these software revisions, STRmix™ Version 2.9, contains a number of new features, including: the introduction of a batch maker mode, allowing multiple interpretations to occur simultaneously and database search templates; memory usage improvements, which are particularly significant in dealing with higher order DNA profile mixtures; and improvements to biological modelling calculations designed to improve the modelling of stutter peaks.

Looking to the future, the STRmix™ team is planning to formulate suitable models for interpreting data as the forensic community embraces new analytical techniques such as massively parallel sequencing (also referred to as “next generation sequencing”). Probabilistic genotyping software for the interpretation of Y-STR (male specific) profiles, which are routinely developed during sexual assault investigations, is also in the development pipeline.