Sales Via Flickr Elliott Brown

Source: Flickr user Elliott Brown.

The market has been slipping, and it's taking high-flying biotech stocks with it. If you're new to biotech investing, that could be a good thing. After all, biotech stocks have tripled the S&P 500's return since 2012, returning an eye-popping 165%, and while that rally's been a boon to longtime biotech investors, it's kept new investors on the sidelines.

Because the current market retreat could be offering up the first solid buy opportunity to pick up biotech stocks on sale, here are two top-tier biotech stocks to buy if they drop.

No. 1: Gilead Sciences (NASDAQ:GILD)
A stable of top-selling HIV medicines and the launch of revolutionary new hepatitis C drugs have resulted in a doubling in sales at Gilead Sciences since 2013. Importantly, leveraging that surge in sales against fixed costs has led to a significant jump in profit that's swelling Gilead Sciences' balance sheet.

Last year, Gilead Sciences' net income hit $12.1 billion, up from $3.1 billion in 2013, and in the first quarter, net income totaled $4.3 billion, up from $2.2 billion a year ago. As a result, despite buying back $3 billion worth of its shares, Gilead Sciences' cash stockpile grew to $14.5 billion exiting the first quarter.

Envy-inspiring growth and a rock-solid balance sheet have led to Gilead Sciences' shares returning more than 20% this year, but more upside is possible. Here's why.

First, many investors held off on buying Gilead Sciences' shares last year out of fear that competition from AbbVie's Viekira Pak would steal market share and lead to a margin-busting price war. However, that hasn't really played out. Despite Viekira Pak's launch and resulting price headwinds, sales of Harvoni -- Gilead Sciences' genotype 1 HCV treatment that Viekira Pak competes against -- surged to $3.6 billion in the first quarter from $2 billion in the prior quarter. Granted, new competing therapies from Bristol-Myers Squibb, Merck & Co, and others remain a concern, but so far, Gilead Sciences is demonstrating that it's still best in class, and I'm willing to give it the benefit of the doubt that it can maintain that leadership.

Secondly, Gilead Sciences' profit has been growing more quickly than its share price, and that's kept a lid on its valuation. Industry watchers think Gilead Sciences' EPS will grow to $11.06 in 2016 (up from expectations for $10.36 90 days ago), and that gives the company a relatively cheap forward P/E just above 10. Any further decline in share price would make this biotech Goliath an arguable steal.

Finally, Gilead Sciences isn't resting on its laurels. Instead, it's investing plenty of money into next-generation therapies that could help drive future sales growth. One of the most intriguing of its research programs involves GS-5816, which is part of a next-generation HCV cocktail the company is working on that could improve non-genotype 1 treatment similarly to how Harvoni revolutionized genotype 1 therapy. Gilead Sciences will report results from four phase 3 trials of GS-5816 this quarter, so investors should keep a close eye out for that data.

No. 2: Celgene Corporation (NASDAQ:CELG)
Another top stock to consider buying on sale is Celgene, which is as successful in treating hematologic cancer as Gilead Sciences is at treating HIV and hepatitis C.

Celgene's best-seller is Revlimid, a widely-used second-line multiple myeloma therapy poised to deliver more than $5 billion in sales this year. Celgene also markets the fast-growing third-line multiple myeloma drug Pomalyst, which had sales of $199 million in the first quarter, up 46% year over year.

In January, Celgene updated investors with a long-term sales forecast that includes Revlimid sales climbing to $7 billion and Pomalyst sales increasing to $1.5 billion in 2017. If that happens, Revlimid and Pomalyst provide investors with plenty of reason to want to own Celgene's shares.

But Revlimid and Pomalyst aren't the only compelling reasons why buying Celgene (especially on sale!) could make sense. Celgene also markets cancer drug Abraxane, which it expects to deliver sales of at least $1.5 billion in 2017, up from $848 million in 2014. And Celgene recently diversified its product line-up with the launch of its first autoimmune drug, Otezla, for the treatment of psoriasis. Otezla's annualized first-quarter sales were just $240 million, but Celgene thinks it could also sell more than $1.5 billion by 2017.

If those growth numbers haven't convinced you that Celgene deserves a spot in your portfolio, you might want to consider this point, too: Celgene's long-term forecast calls for its total revenue to double to more than $20 billion by 2020.

Still not convinced? Then I'll leave you with this final argument: Celgene's profit-fueled balance sheet is opening a bunch of doors for future sales growth thanks to collaboration deals like its recent $1 billion collaboration with immuno-oncology leader Juno Therapeutics on CAR-T research.

Tying it together
Biotech stocks are inherently volatile, and a range of risks, including pipeline uncertainty, mean that's unlikely to change. Reducing those risks by focusing on top-shelf industry leaders like Gilead Sciences and Celgene Corp makes a lot of sense -- especially if you're one of those investors who have been waiting for a market dip to buy.

Todd Campbell owns shares of Celgene and Gilead Sciences. Todd owns the equity research firm E.B. Capital Markets, LLC. E.B. Capital's clients may have positions in the companies mentioned. The Motley Fool recommends Celgene and Gilead Sciences. The Motley Fool owns shares of Gilead Sciences. Try any of our Foolish newsletter services free for 30 days.

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