Home DNA test results from the 5 million customers of 23andMe will now be used by drug giant GlaxoSmithKline to design new drugs, the two companies announced Wednesday.

It’s the biggest partnership yet aimed at leveraging the increasingly popular home genetic testing market, in which customers pay for mail-in saliva tests that are analyzed by various companies. 23andMe dominates the market.

“By working with GSK, we believe we will accelerate the development of breakthroughs,” 23andMe CEO Anne Wojcicki wrote in a blog post.

23andMe patrons are asked if they want to participate in scientific research. The new agreement moves this consent firmly into the field of active drug discovery research.

“As always, if our customers do not want to participate in research, they can choose to opt out at any time,” Wojcicki wrote.

She emailed 23andMe customers after the announcement, including a quick link to opt out of the research.

Glaxo has invested $300 million in 23andMe and the companies have a four-year deal that gives Glaxo exclusive rights to collaborate with the DNA testing company to develop drugs.

Peter Pitts, president of the Center for Medicine in the Public Interest, said the companies should pay the 23andMe customers whose DNA is used in any research.

"Are they going to offer rebates to people who opt in so their customers aren’t paying for the privilege of 23andMe working with a for-profit company in a for-profit research project?" he asked.

"It's one thing for NIH (the National institutes of Health) to ask people to donate their genome sequences for the higher good," Pitts told NBC News.

"But when two for-profit companies enter into an agreement where the jewel in the crown is your gene sequence and you are actually paying for the privilege of participating, I think that's upside-down."

Pitts also questioned whether there were solid protocols for protecting the privacy of 23andMe customers.

The first project will look at possible new drugs for treating Parkinson’s disease, based on a gene called LRRK2 that is mutated in some Parkinson’s patients. A study released Wednesday found that the gene may play a significant role in Parkinson’s even among patients who don’t have mutations.

Glaxo is already working on drugs that might work based on LRRK2 activity.

The partnership was dreamed up by the companies’ two chief scientific officers: Hal Barron of Glaxo and 23andMe’s Richard Scheller. The two previously worked together at another drug company, Genentech, they told CNBC.

“When you get a genetically validated target and you pursue it, it’s twice as likely to end up being a medicine,” Barron said in an interview on CNBC.

“The over 5 million customers that 23andMe has gained access to is really many larger ... 10 times larger, than some of the other databases out there,” he added.

One of the big obstacles to genetics research is getting enough people to donate their DNA and paying to sequence it. The 23andMe database delivers a huge number of customers who have already consented and whose DNA has already been partly sequenced.

The company can go back and do more sequencing on people who have genetic variations that are of interest.

“We are also excited to leverage the patients, to have them be part of this drug discovery process,” Barron said.

23andMe has been doing some of its own drug development and will now share that information with Glaxo under the agreement.