DNA Mixture Interpretations: A Q&A With NIST’s John Butler

Whether from skin cells, saliva, semen or blood, DNA from a crime scene is often collected and tested in a lab to see if a suspect’s DNA is likely a contributor to that sample or not. But every DNA sample tells a different story, and some samples are easier to interpret than others. The simplest type of DNA profile to interpret is one where the sample includes hundreds of cells from only one person. When two or more people have contributed to a sample, it’s called a DNA mixture. Some mixtures are so complicated that their stories remain a mystery even to the best forensic DNA experts. John Butler, a Fellow at the National Institute of Standards and Technology (NIST), and a team of authors have recently completed a draft scientific foundation review of the different methods forensic laboratories use to interpret DNA mixtures. The team urged for more interlaboratory participation in studies to demonstrate consistency in methods and the technology used in DNA mixture interpretation, as well as a need for sharing data publicly. In this interview with NIST’s Christina Reed, Butler — who has over 30 years of experience with DNA profiling, is the author of five books on the subject, and has led training workshops on interpreting DNA mixtures — answers some basic questions about the importance of this fast-growing field of forensic science.

What’s the concern here? Why is NIST reviewing how labs are conducting DNA mixture interpretations?

Why is it important that this data be public? Why is transparency and openness important in science?

Why are forensic scientists now frequently dealing with casework that requires analyzing DNA mixtures in their labs?

What can an analyst do to deal with complications?

What do you hope this report will accomplish?

This post originally appeared on Taking Measure, the official blog of the National Institute of Standards and Technology (NIST) on July 28, 2021.

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