Source: Flickr user Stockmonkeys.com.

Biotech stocks have been among the market's brightest shining stars, but many of these stocks have already made big moves higher. As a result, investors could be wondering if there are any companies in the group still worth adding to their portfolios. With that in mind, here are two companies that I think can still be stashed away in portfolios for the long haul.

Growth on the cheap
First up is Gilead Sciences (NASDAQ:GILD), a biotech powerhouse that's the dominant manufacturer of medicine used to treat HIV and hepatitis C. The company boasts five different billion-dollar blockbuster HIV therapies, including the fast-growing combination drugs Stribild and Complera. As a result, Gilead Sciences notches more than $10 billion in sales annually from its HIV medicines.

Last year, Gilead Sciences expanded its product portfolio beyond HIV with the launch of its next-generation hepatitis C drugs Sovaldi and Harvoni. Those two therapies, which revolutionized HCV treatment by boosting cure rates and cutting treatment duration, became instant blockbusters, generating more than $12 billion in combined sales last year.

Despite Gilead Sciences' gold-standard leadership in HIV and HCV, investors have been concerned that competitors could eat into the company's market share -- particularly in hepatitis C. This has kept a lid on Gilead Sciences' valuation -- and that may mean that its shares still offer plenty of upside.

Currently, investors are paying only 13.4 times trailing 12-month earnings and 10.6 times forward 12-month earnings to buy Gilead Sciences' shares. That could be a bargain because Gilead Sciences is rolling out a new variation of its long-standing HIV drug Viread that extends the life of its HIV patent portfolio, and the company is on tap to report data from trials evaluating a next-generation pan-genotype hepatitis C drug that could solidify its dominance in treating HCV.

Source: GPhrma.

Envy-inspiring future growth forecasts
Although cancer and autoimmune disease drugmaker Celgene Corp.'s (NASDAQ:CELG) shares aren't trading as cheaply as those of Gilead Sciences, Celgene's long-term sales and profit forecasts suggest that its current share price looks very reasonable.

In January, Celgene announced that its sales and profit could reach $13 billion and $7.50 per share, respectively, in 2017, up significantly from the $9 billion and $4.60 per share that Celgene expects to report in 2015. Celgene also offered up a forecast for 2020 that calls for sales and EPS of at least $20 billion, and $12.50 per share, respectively.

Celgene's January guidance already made its shares intriguing to investors, but Celgene got even more attractive last month when it boosted its January long-term forecast after announcing a $7.2 billion acquisition of the multiple sclerosis drug-developer Receptos. After factoring in potential sales tied to Receptos' oral MS drug, ozanimod, Celgene thinks sales and EPS could eclipse $21 billion and $13 in 2020.

Celgene's rosy outlook could even prove to be too conservative. Celgene thinks that ozanimod's peak annual sales could reach between $4 billion and $6 billion, and additional opportunity for revenue and profit could come from collaborations with promising clinical-stage companies such as Juno Therapeutics, which is working on immuno-oncology drugs targeting blood cancer.

Tying it together
There's no guarantee that Gilead Sciences and Celgene won't stumble along the way, but both of these companies are top-notch biotech leaders with needle-moving drugs under development, rock-solid balance sheets, and arguably inexpensive price tags that I think are worthy of including in long-term portfolios.