Here's something that might sound strange: There are companies now that print and sell DNA.

This trend — which uses the term "print" in the sense of making a bunch of copies speedily — is making particular stretches of DNA much cheaper and easier to obtain than ever before. That excites many scientists who are keen to use these tailored strings of genetic instructions to do all sorts of things, ranging from finding new medical treatments to genetically engineering better crops.

"So much good can be done," says Austen Heinz, CEO of Cambrian Genomicsin San Francisco, one of the companies selling these stretches of DNA.

But some of the ways Heinz and others talk about the possible uses of the technology also worries some people who are keeping tabs on the trend.

"I have significant concerns," says Marcy Darnovsky, who directs the Center for Genetics and Society, a genetics watchdog group.

A number of companies have been taking advantage of several recent advances in technology to produce DNA quickly and cheaply. Heinz says his company has made the process even cheaper.

"Everyone else that makes DNA, makes DNA incorrectly and then tries to fix it," Heinz says. "We don't fix it. We just see what's good, what's bad and then we use the correct pieces."

The company does that by putting chunks of their DNA on tiny metal beads that emit different colors. That lets a computer scan millions of pieces of DNA to find the right ones.

"So we just take a picture, change a filter, take a picture, change a filter, take a picture, change a filter. And we read the sequences," he says.

It's basically a high-tech version of a spell-checker.

Then Cambrian chooses and "prints" the correct stretch of DNA by firing a computer-controlled laser beam at a glass tray holding millions of these tiny metal beads, each one coated with DNA. The impact of the laser propels the bead carrying the correct DNA into a tray.

"The DNA laser 'printer' is essentially a sorter," he says. It can produce any strand of DNA, made to order, and Heinz can crank out a lot of DNA this way.

So far, the company's main customers are drug companies, which use the strings of DNA Cambrian Genomics makes to do things like genetically engineer microbes to try to find new medicines.

"They may be interested in making a protein that attacks a cancer cell with some kind of killer payload," he says.

Other users are genetically engineering plants to try to make them grow better. But Heinz envisions a day when mass-produced DNA can genetically engineer people — or let anyone use DNA like computer code to design their own organisms.

"I think some people will find the process of designing and making organisms just fun, in and of itself," he says.

But this sort of talk makes some people nervous.

"Heinz talks openly about everybody being able to create entirely novel creatures," Darnovsky says. "Is that what we want? Do we want anybody, including potential terrorists, to be able to create entirely novel life forms — new creatures? Do we want the teenager next door to be creating Godzilla in the bathtub? I don't want that."

She also worries about genetically engineered plants running amok, ruining the environment. And, she says, genetically engineering people would be even worse.

"Many of the figures in the synthetic biology field are not shy at all about embracing that prospect that we're going to use synthetic biology to redesign humanity and to engineer the traits in our children," she says. "And that I find extremely disturbing."

But others say those kinds of fears are exaggerated.

"Like every other technology, we need to be paying attention to how it's used," says Rob Carlson, a biotechnology analyst at Seattle-based Biodesic. But "it is not intrinsically more dangerous than other technologies," he adds. "And, in fact, if you wanted to do harm, there are many easier ways to go about causing harm than using synthetic DNA."

Heinz says his company is being very careful. It won't sell DNA to just anyone. And the potential benefits to society, he thinks, are huge.

"We can make DNA that would be used to make a virus that could target your cancer cells. And I think it can be helpful for dealing with some of the problems that humans have created. If we can make plants that can suck more carbon out of the atmosphere, we can deal with global warming," he says.

In addition, Heinz says he thinks "in general most people want children that are healthier than they were — maybe better. I think as a race, or as a species we have a goal of improving who we are."

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