US prosecutors say they have a new tool to persuade jurors in cases involving twins: cutting-edge DNA testing that can identify genetic mutations to differentiate between identical siblings.
Boston prosecutors re-indicted a rape suspect earlier this month after receiving new DNA test results they say pointed to him but ruled out his twin in two rape cases in 2004.
Dwayne McNair, of Dedham, pleaded not guilty on Monday and is being held on $US500,000 ($A540,980) bail.
Boston prosecutors had a similar case a decade ago. Other cases have popped up in other parts of the US, showing the challenges of prosecuting a twin.
Standard DNA testing has not been able to differentiate between identical twins because they come from a single fertilised egg.
"The science behind this testing has been used in research, in paternity tests and in a variety of other capacities," said Jake Wark, a spokesman for Suffolk District Attorney Daniel Conley, whose office is now prosecuting McNair.
"The forensic application is novel, but it's supported by a widely accepted scientific foundation," Wark said.
McNair was scheduled to go on trial last April. But after prosecutors heard about the new DNA test done by a German company, Eurofins Scientific, they decided to temporarily withdraw the charges against McNair so they could get a new round of testing done on DNA taken from a condom used in one of the rapes. Eurofins says it has been able to identify genetic mutations to differentiate between twins.
Prosecutors re-indicted McNair earlier this month after the new test results pointed to McNair and ruled out his twin brother, Wark said.
Boston prosecutors have been through this before. In 2004 and 2005, two different juries were unable to reach verdicts in a rape case against another twin, Darrin Fernandez.
Several jurors later said they could not reach unanimous verdicts because DNA found on the victim's pillowcase could have come from either Fernandez or his twin brother.
Fernandez was convicted after a third trial.
Authorities in other states have also struggled with prosecuting twins in cases involving DNA evidence.
In Colorado, a former US Army officer blamed his twin brother for a string of sexual assaults on young girls.
Investigators said DNA linked Aaron Lucas to the abduction of an eight-year-old girl in Colorado Springs and to other cases. Lucas's twin denied involvement, and Lucas ended up pleading guilty in November in nine cases ranging from indecent exposure to kidnapping and rape. He was sentenced to 20 years to life in prison.
In Grand Rapids, Michigan, months of DNA testing was unable to determine which of two identical twins raped a woman in 1999. Neither brother was charged in the case.
Earlier this year, police in Grand Rapids said they planned to send new DNA samples from the brothers to a DNA analysis company in Greenville, North Carolina, which also says it can look at genetic mutations to differentiate between twins.
After the new DNA tests were conducted in the McNair case, prosecutors said the results showed that McNair was two billion times more likely to have been the source of the DNA evidence than his twin.
McNair's lawyer, Robert Tobin Jr, questioned the reliability of the new testing and said he will challenge the prosecution's attempt to put it before a jury.
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