On 21 August, a jury found Russell Marubbio, 54, guilty of a sexual assault that occurred in Woodbridge, Virginia, in 1987.
Virginia-based Parabon NanoLabs ran an analysis on the DNA sample found at the scene nearly four decades ago. Genealogist CeCe Moore then traced the perpetrator’s family tree and seemed to hit a wall when the sample led to identical twins.
Usually, that would be the end of using DNA to solve the case. Identical twins have the same DNA – almost. There are sometimes a few cells that split and evolve differently.
"On average, twins will have eight of those types of differences across the whole 3 billion bases" of the genome, said Ellen Greytak, the director of bioinformatics at Parabon.
The scientists were able to detect some of these mutations in the sample, compared to cheek swabs from the Marubbio brothers, leading to the conviction.
"This landmark case marks the first successful admission and application of this specific technique in a US court to overcome challenges in DNA identification of identical twins," Amy Ashworth, the commonwealth’s attorney for Prince William county, Virginia, wrote in a statement.
On 19 December 1987, a 50-year-old cashier at a Chevron gas station in Woodbridge, took a bathroom break and was followed by a young man. He restrained her and sexually assaulted her at knifepoint.
The assailant left semen at the crime scene, but his DNA never matched with any offenders listed in the law enforcement database known as the Combined DNA Index System (Codis).
In 2019, Giannina Pinedo, a master detective with 22 years of experience, transferred to the cold case unit in Prince William county. She began looking through unsolved cases where DNA samples were present at the crime scene, alongside Colleen Grantham, another master detective with 23 years of experience.
Scientific and technological advances in DNA sequencing and genealogical databases are now yielding breakthroughs in cases that long ago went cold. The Golden State Killer, for example, was arrested in 2018 after genetic genealogy led to a suspect, Joseph James DeAngelo, who pleaded guilty in 2020.
The two Virginia detectives worked with several labs in different states. They submitted the DNA evidence to Parabon in August 2022. The company put together a sketch, based on genetics, of what the perpetrator might look like – and Moore, their chief genetic genealogist, tracked down possible suspects.
The detectives worked with special agents in Florida, where the Marubbio twins lived, to obtain cheek swabs from the brothers for genetic sequencing.
Parabon scientists then did whole-genome sequencing to uncover the smallest genetic differences between the men.
"These are somatic mutations," said Claire Glynn, a professor and the executive director of the Henry C Lee college of criminal justice and forensic sciences at the University of New Haven. "A better word, I think, for the public is acquired mutations, where you can differentiate between identical twins."
She thinks of it as similar to punctuation in a sentence. The "words" don’t change – the arrangements of A, T, G and C are the same in both twins – but sometimes there are commas in one sample where another has periods.
"We would see the mutations from very early on in life," Glynn said. They develop in utero, after the embryo splits in two.
"In terms of the science, that’s well studied and well understood," she said. "The science behind it is absolutely robust. It’s simply that it’s not that common that we would encounter this in cases."
There was a case in Massachusetts in 2017 where a similar analysis was performed, but the judge on that case said the evidence was not admissible.
"Occasionally, you get these identical twin defense cases. And it used to be, they would give up," said David Kaye, a professor emeritus of law at Arizona State University. "They would say: 'Well, all right, the DNA is not going to help us.' Now, it is possible, and it will."