The FDA just approved another gene-altering cancer treatment that will save thousands of lives. Yescarta from Gilead Sciences (NASDAQ:GILD) teaches cancer patients' immune cells to seek out and destroy cancer cells with so much fervor that oncologists can hardly contain their excitement.

A pipeline full of similar therapies, and the means to develop them, earns Gilead a spot on this list, but you should know that today's best gene therapy stocks aren't limited to the field of cancer. While bluebird bio, Inc. (NASDAQ:BLUE) has cancer therapies in development, its experimental treatment for a rare chromosomal disease is barreling toward an approval. Finally, AveXis, Inc. (NASDAQ:AVXS) has taken aim at a muscle-wasting disorder with impressive results.

Stethoscope on top of fanned-out $100 bills.

Careful monitoring of the gene therapy space turned up a few stocks that could make you rich. Image source: Getty Images.

All three of these stocks have risen sharply in recent weeks. Here's why they could go much further in the years ahead.

1. Gilead Sciences, Inc.: A leader overnight

This might be the second company to earn an approval for a chimeric antigen receptor T-cell (CAR-T) therapy, but it wasn't even on the radar a couple of months ago. Gilead earned its place on this list practically overnight by scooping up Kite Pharma for $11.9 billion, which was nearly triple the company's value at the beginning of the year.

Gilead will soon begin recording sales for recently approved Yescarta, formerly axicabtagene ciloleucel. The therapy inserts genes into a patient's T-cells that send them on a seek-and-destroy mission, with eye-popping results for lymphoma patients that have exhausted previous treatment options. Not long ago, oncologists would have been impressed if any of these patients responded to an experimental therapy. During trials supporting Yescarta's approval, though, a stunning 51% of those given a single infusion had no detectable disease remaining months later. 

While the results are thrilling, a successful launch is far from guaranteed. New cancer therapies often cost more than $100,000 for a year of treatment, and once you factor in hospitalization expenses for patients who don't respond, Gilead's $373,000 list price for a single Yescarta infusion could save end payers a bundle in the long run. Enough patients are eligible for Yescarta that peak annual sales of the therapy could top out around $2 billion if private insurers and other end payers agree with Gilead's value proposition.

Although Gilead's ability to squeeze out a profit from its $11.9 billion acquisition of Kite Pharma is far from guaranteed, you'll be glad to know this stock could still looks like a bargain, even if excitement for CAR-T fizzles. At recent prices, this biotech stock's market cap is just 8.1 times the $13.4 billion in free cash flow its highly profitable operations generated over the past year.

Person handing three $100 bills to a doctor.

Pricey CAR-T therapies present a long-term value proposition end payers might not recognize. Image source: Getty Images.

2. bluebird bio, Inc.: Multiple shots on goal

Unlike Gilead Sciences, this biotech has no approved products to sell yet. Total dependence on experimental gene therapies still in development makes this stock a great deal riskier than Gilead's, but a couple of recent developments lifted Bluebird's chances of success.

Bluebird's most advanced candidate, called Lenti-D, uses a virus to inject functional copies of mutated genes that cause cerebral adrenoleukodystrophy (CALD). Without treatment, patients born with the disease nearly always succumb to major neurological disabilities within a couple of years. Two years after receiving an infusion of Lenti-D, though, 15 of 17 CALD patients were free of major disabilities.

Although Lenti-D might not become a blockbuster drug on its own, it could go a long way toward funding development of a similar gene therapy candidate with far more potential. LentiGlobin could be a huge improvement that allows huge swaths of people with sickle-cell disease and beta-thalassemia. Peak sales estimates for the therapy, which allows patients to produce functional hemoglobin, run as high as $4 billion annually, and there's still more.

Bluebird is also developing CAR-T therapies in partnership with Celgene (NASDAQ:CELG). This summer, the partners thrilled investors with news of a 100% response rate among 15 advanced-stage multiple myeloma patients treated with a single dose of bb2121. Observation periods that range between eight and 54 weeks makes the data difficult to interpret, but a total lack of disease progression among these advanced-stage patients is highly encouraging.

Celgene has since licensed bb21217 from Bluebird, which should show even more anti-tumor activity than its precursor. While Celgene will put up the cash for expensive mid- to late-stage development, Bluebird remains eligible for hefty milestone and royalty payments if one of these partnered CAR-T therapies continues to succeed.

Although bluebird bio could make you rich, the shares have a long way to fall if catastrophe strikes. The company's recent market cap of about $6.4 billion is awfully high for a business without a product to sell. That said, a major partnership and diverse candidate lineup make this gene therapy look far less risky than most of its peers. 

3. AveXis Inc.: Still batting a thousand

A few months ago, a 100% success rate for this company's lead candidate was a big reason I believed the stock's rally would continue, and results from the latest follow-up are still pointing in the right direction. Few infants diagnosed with spinal muscular atrophy (SMA) ever develop an ability to swallow, and most eventually lose the strength to breathe unassisted before their first birthday. In January, AveXis thrilled investors with news that 15 of 15 patients treated with two different doses of the company's lead candidate, AVXS-101, were still breathing on their own at 13.6 months of age. At a follow-up seven months later, all of the patients continued breathing without a respirator, plus nine of 12 who received the higher dose were also able to sit unassisted.

With results this impressive, Biogen (NASDAQ:BIIB) has plenty to worry about. The big biotech launched the first ever SMA treatment at the beginning of the year, and sales of the drug are already on pace to reach $1 billion in its first year. AveXis intends to begin a trial designed to support an application before the end of 2017, which means Spinraza sales should be safe for a few more years.

If AVXS-101 continues to impress, Biogen or one of its peers will more than likely make an attractive offer that would enrich AveXis shareholders. Unlike Gilead or Bluebird, though, essentially all of AveXis Inc.'s value is riding on the only candidate it has in clinical trials at the moment. Although everything has gone swimmingly so far, shocking upsets do happen. Although this stock might have more potential upside than Gilead Sciences, a recent market cap of $3.2 billion is an awfully tall perch to fall from.

Cory Renauer owns shares of Gilead Sciences. The Motley Fool owns shares of and recommends Biogen, Bluebird Bio, Celgene, and Gilead Sciences. The Motley Fool has a disclosure policy.