Hernandez is on trial for murder in the June 2013 killing of Odin Lloyd, who was dating the sister of Hernandez's fiancee. Lloyd's body was found in an industrial park near Hernandez's home, and prosecutors have said previously a marijuana cigarette was found near his bullet-riddled body.
Diane Fife Biagiotti of the state police crime lab told jurors that she received rolling paper from a five centimetre-long cigarette butt to test for DNA. She said she found it had the DNA of at least two people on it.
She said she first compared it with Lloyd's DNA and discovered he could have been one of the people whose DNA appeared on the butt. She said she then was able to use Lloyd's DNA to deduce the second DNA contributor.
"I did a comparison between the profile I deduced and the profile of Aaron Hernandez," she said. "I found that the profile from Aaron Hernandez matched the deduced DNA profile."
She then ran a statistical analysis and determined the likelihood it was someone else is one in more than a quadrillion.
"How many zeros does a quadrillion have?" prosecutor William McCauley asked her?
"One with 15 zeros after it," she replied.
He then asked her the world's population: seven billion — and pointed out a quadrillion is one million times a quadrillion.
But Hernandez lawyer James Sultan went after other DNA evidence in the case, including some collected from a spent shell casing found inside a car the former New England Patriot rented. Police found the casing in a trash bin at an Enterprise rental car, stuck to some blue bubble gum. An Enterprise employee said she found both items under the driver's seat of the car and threw them away.
Biagiotti testified she found Hernandez DNA on the shell casing, but she also acknowledged she did not know the gum had been stuck to the shell before she did her test.
"Would you agree with me, Ms. Biagiotti, that there is a high likelihood that the DNA contained in the saliva on a chewed blue chewing gum would be transferred to that shell casing?" Sultan asked.
"Yes, I would agree with that," she said.
She also acknowledged that it is uncommon to find a testable amount of DNA on a spent cartridge casing and that the lab had never tested the chewing gum for DNA. Sultan also asked her if she had received a DNA profile for Hernandez co-defendant Ernest Wallace to compare it with evidence in the case. She said she had not.
Wallace and another man, Carlos Ortiz, have also been charged in the killing. They have pleaded not guilty and will be tried separately.