Source: Generic Pharmaceuticals Association.

If you're interested in investing in the biotechnology industry, you're not alone. The allure of profiting from aging baby boomers, an increasingly insured population, and innovative gene therapies has made biotechnology one of the market's top-performing industries.

However, investing in biotechnology stocks can be tricky. They can move wildly based on rumors and the latest clinical trial news, so for most investors it is likely best to stick with best-in-breed companies. With that in mind, here are three industry Goliaths that investors should be consistently tracking.

No. 1: Gilead Sciences (NASDAQ:GILD)
Gilead Sciences has been at the forefront of research into treating some of the planet's toughest diseases, including HIV and hepatitis C. A steady stream of therapies for these indications has turned the company into the market share leader for both indications, with over $25 billion in global sales last year.

Gilead's domination isn't likely to end soon, given that the company is investing big money into next-generation treatments that can protect its market share.

In HIV, Gilead Sciences has reformulated its top-selling drug Viread to improve its safety, and that opens up opportunities to create new multidrug combination therapies. The company is also conducting early stage work on a therapy that it hopes could one day create a functional cure for the disease by eliminating HIV's ability to hide from medicine that targets the virus.

In hepatitis C, Gilead has launched two multibillion-dollar drugs -- Sovaldi and Harvoni -- that are reshaping treatment for 150 million people worldwide. Both drugs already offer cure rates north of 90% for many, but Gilead Sciences hopes to improve upon that success by delivering similar cure rates to all patients with shorter treatment regimens. In the third quarter, the company is expected to release data for a pan-genotype combination of Sovaldi and GS-5816 that could significantly improve therapy for non-genotype 1 hepatitis C patients. Meanwhile, studies are evaluating a therapy that could reduce treatment from the current standard of eight to 12 weeks down to six weeks, or less.

Also of note for investors are Gilead's efforts to expand into new indications, such as cancer and nonalcoholic steatohepatitis, a liver disease that is a major cause of liver failure and transplant.

No. 2: Celgene Corp. (NASDAQ:CELG)
Celgene should be on investors' radar because it markets two of the planet's best-selling blood cancer medications and because it expects to post impressive sales growth over the coming years.

Celgene's Revlimid is the market share-leading second-line treatment for multiple myeloma; it generated nearly $5 billion in sales last year. After the drug won FDA approval in February for use as a first-line treatment, Celgene believes Revlimid's sales will jump to $5.6 billion-$5.7 billion this year.

Celgene also markets Pomalyst, a third-line multiple myeloma drug that launched in 2013 and generated sales of $199 million in the first quarter, up 46% from a year ago.

Additionally, Celgene markets the cancer drug Abraxane and the psoriasis drug Otezla, and it has a deep pipeline that could produce other new treatments. For instance, Celgene reported intriguing midstage results for GED-0301 for the treatment of Crohn's disease two weeks ago, and last week the FDA granted fast-track status to Celgene's luspatercept for use in beta-thalassemia major patients.

Since Celgene expects that overall sales to jump from $7.7 billion last year to at least $13 billion in 2017 and $20 billion in 2020, its shares rate must-watch status.

No. 3: Amgen (NASDAQ:AMGN)
A slate of top-selling cancer and autoimmune disease drugs has turned Amgen into one of the best-known biotech stocks on the market. For three reasons, it is one to watch.

First, Amgen has generated billions of dollars in sales from the blood cell-boosting drugs Neulasta and Neupogen, but expiring patents and approvals of generic biosimilars could take a toll on sales. With the FDA approving Novartis' Neupogen biosimilar in March, investors should watch Amgen to see how much market share it loses to these new generic competitors.

Second, Amgen plans to blunt the risk of generic competition by entering the biosimilars market itself. The company is developing nine biosimilars that it believes could generate sales of at least $3 billion annually.

Third, Amgen's Repatha could move the sales needle significantly. The company is awaiting an FDA decision on use of Repatha as a cholesterol-lowering therapy. Since statins are the current standard of care and they're among the most widely prescribed drugs in America, the market for Repatha could ultimately be worth billions of dollars. For that reason, investors ought to pay close attention to the FDA's decision on Aug. 27, and, if Repatha is approved, to how Amgen's marketing efforts pan out.

Tying it together
Biotech is an incredibly intriguing industry, but it is also fraught with risk.

By focusing on leaders such as Gilead Sciences, Celgene, and Amgen, investors can reap the benefits of rising demand for next-generation therapies with fewer risks than they might have in owning clinical stage companies. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.