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In 2017, the gene editing revolution for agriculture kicked off when a St. Paul, Minnesota-based startup company called Recombinetics edited the genomes of dairy cattle to create animals that never grew horns. CEO Tammy Lee Stanoch hailed the breakthrough as a new era in precise breeding of farm animals. 

But as MIT Technology Review reports, Stanoch may have spoken too soon. US Food and Drug Administration scientists have taken a closer look at the genome sequence of one of the edited animals, a bull named Buri, and have found that its genome contains a stretch of bacterial DNA that includes an antibiotic resistance gene.

This addition of DNA from a different species occurred during the gene editing process and the company never detected it, MIT Tech Review says. To make the cows hornless, Recombinetics took skin cells from Holstein dairy bulls and used plasmids to convey TALENS carrying editing instructions for the genetic change into the cells. The editing involved swapping out about 200 genetic letters, MIT Tech Review says, and the plasmids were supposed to be temporary. The edited skin cells were then used in a cloning procedure to generate two new animals that were copies of the original bulls but didn't grow horns.

Recombinetics had claimed the animals were 100 percent bovine and even tut-tutted the FDA for wanting the regulate the animals. This is a setback for the company, MIT Tech Review notes. Recombinetics has created prototypes of other gene-edited animals, including heat-resistant cattle and pigs that never hit puberty. It's also a setback for efforts to make gene editing a routine procedure in the breeding of farm animals, especially since Recombinetics has been loudly objecting to any oversight by the FDA. 

The company never pursued formal FDA approval for the hornless cattle in the US, in part because it disagreed with the notion of regulation, MIT Tech Review says. However, its collaborator at the University of California, Davis, veterinary scientist Alison Van Eenennaam, opened a file on the animals with the FDA last year. She was curious to know whether the agency would approve the gene-edited animals to be slaughtered for food.

That's when an FDA bioinformatician running tests on the bulls' genomes found the plasmid, which had integrated into Buri's genome, according to the article. The presence of bacterial genes classfies the animals as GMOs. 

It's not clear if the bacterial DNA poses a larger risk, and it's unlikely to affect the bull or any people who might eat it. But the antibiotic resistance gene could be taken up by any of the other bacteria present in the animal's gut or body, MIT Tech Review says.

But the animals are definitely out of the running for regulatory approval, now, the article adds. Regulators in Brazil have already rejected a revised petition by Recombinetics connected to the animals.