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With the stock market in nearly nonstop rally mode over the past six years, investors haven't needed to look far to uncover an abundance of growth stocks. But not all growth stocks are created equal: While some could still deliver extraordinary gains, others appear considerably overvalued and might instead burden investors with hefty losses.

What exactly is a growth stock? Though it's arbitrary, I'll define a growth stock as any company forecast to grow profits by 10% or more annually during the next five years. To decide what's "cheap," I'll use the PEG ratio, which compares a company's price-to-earnings ratio to its future growth rate. Any figure around or below 1 could signal a cheap stock.

Here are three companies that fit the bill.

AbbVie Inc.

We'll begin the week in megacap territory by looking at the $105 billion drug giant AbbVie (ABBV 0.92%).

What investors needs to understand about AbbVie, and any drugmaker for that matter, is that branded therapeutics are protected by patents for only a finite amount of time. Eventually, generic drugs enter the marketplace and disrupt the revenue stream of branded therapies. This is a bit concerning, because AbbVie's leading drug, Humira, an anti-inflammatory drug with 10 approved indications, is currently the best-selling drug in the world. Humira generated more than $14 billion in sales for AbbVie in 2015, and through the first-half of the year has accounts for $7.73 billion of AbbVie's $12.39 billion in revenue.  When Humira's protection period is over, AbbVie and its shareholders could feel some pain.

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Now the good news is AbbVie has an intriguing pipeline and has made some moves from an M&A perspective that should help ease its pain when Humira gets exposed to biosimilar or generic competition. For what it's worth, AbbVie's management remains confident that it can hold off biosimilar competition from entering the marketplace until 2022, which would be great considering its exceptional pricing power and cash flow from the product.

On the research and development front, AbbVie is counting on its oncology therapeutics and own biosimilar research to lead the charge. AbbVie has a dozen oncologic candidates in clinical trials, some of which are already approved and on pharmacy shelves. For example, Venclexta, a first-to-market BCL-2 inhibitor developed by AbbVie and Roche, is already approved to treat relapsed chronic lymphocytic leukemia with the 17p deletion, and it's being studied in five additional blood cancers. While peak annual sales estimates vary wildly, seemingly all Wall Street analysts seems to believe Venclexta sales will soar well past $1 billion annually.

On the acquisition front, AbbVie gobbled up Pharmacyclics last year in a $21 billion deal to gain hold of a good chunk of blood cancer drug Imbruvica's revenue stream. Imbruvica was developed by Pharmacyclics with the help of Johnson & Johnson. AbbVie's CEO Richard Gonzalez is standing by his estimate that Imbruvica could generate $12 billion in peak annual sales, which would definitely take the sting off weakening Humira sales in the years to come. AbbVie's share at peak sales would be about $7 billion, annually.  

Currently sporting a PEG ratio of 0.8 and a healthy 3.5% dividend yield, AbbVie could be just what the doctor ordered.

Sibanye Gold Ltd.

Next, we're going to dig deep into our chest of undiscovered mid-cap stocks and pull out Sibanye Gold (SBSW -0.94%), an underground and surface gold miner operating out of South Africa.

As with all African gold miners, two issues are a constant concern. First, there always seem to be concerns about political instability; however South Africa tends to be one of the few exceptions to the region. Secondarily, labor in South Africa tends to be more expensive and prone to disputes or strikes. A third issue for all gold miners to concern themselves with is the physical price of gold itself. If gold prices fall, African miners, which tend to have higher all-in sustaining costs, are among the first to feel the pain.

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With those concerns in mind, let's take a gander at why Sibanye Gold could be a growth stock to consider owning.

To begin with, physical gold's strength has shown little sign of slowing. The opportunity cost of owning gold remains low thanks to historically low lending rates in the U.S. and other developed countries around the world. Unless we see multiple interest rate hikes in the U.S., gold is likely to remain an attractive investment relative to bonds or CDs. Also, a recent World Gold Council report observed that demand for gold increased 15% in the first-half of 2016 while supply rose by just 1%. The fundamental picture for gold remains strong.

Individually, most of Sibanye Gold's metrics are trending in the right direction. Favorable dollar-rand exchange rates helped push its all-in sustaining costs down 20% to $908 an ounce in the first-half of 2016, all while the company maintained its annual guidance and generated a 128% increase in operating profit to $351 million. Gold production also increased by 5% from the first-half of 2015. Incredibly, Sibanye delivered these phenomenal results despite multiple operating disruptions. Looking ahead, its management team anticipates gold production topping 1 million ounces annually through 2028, with the life of its gold production extending beyond 2040.

Valued at a mere six times forward earnings and with a sub-1 PEG, Sibanye could be the lustrous growth stock you've been looking for.

PennTex Midstream Partners LP

To hit all ends of the spectrum, we'll end by taking a look at an under-the-radar small-cap in the energy sector, PennTex Midstream Partners (NASDAQ: PTXP). PennTex is a midstream acquirer of energy infrastructure assets in northern Louisiana that gathers and processes natural gas and natural gas liquids.

The biggest worry for investors in a midstream provider like PennTex is what might happen to the underlying assets it gathers and processes. If natural gas and/or natural gas liquid prices fall, it could reduce the incentive for drillers to produce. Less production could equal less in the way of future contracts, and a potentially murkier future for PennTex.

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Though that scenario might be somewhat believable, there are a host of reasons growth investors are probably going to like PennTex Midstream Partners.

The first reason to like PennTex is that the majority of its revenue is generated from take-or-pay contracts. These contracts provide something of a financial floor under PennTex, guaranteeing it revenue for its gathering and processing services whether the product gets delivered or not. But what investors may like even more is that PennTex's largest customer increased its minimum volume commitments from 345,000 MMBtu per day during Q2 2016 to 460,000 MMBtu per day. These contracts, barring bankruptcies and abrupt cancellations, provide downside protection for midstream companies like PennTex. 

Secondly, the U.S. Energy Information Administration earlier this year predicted that global energy consumption would increase by 48% between 2012 and 2040. Though renewable=energy sources are expected to grow faster than any other source of energy, natural gas consumption could grow by nearly 100 quadrillion Btu's annually by 2040 from 2012 levels, outpacing growth from other fossil fuels. That bodes well for U.S. shale production and PennTex's long-term demand.

Lastly, there's no shame in growth investors basking in the above-average yields of midstream limited partnerships. PennTex is paying out $1.14 on an annualized basis, which works out to a delectable 6.6% yield. Top this off with a PEG of just 0.7, and you have what could be a hidden growth gem in the energy sector.