Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to invest in companies where insiders have been snapping up shares but don't have the time to track them down, the Guggenheim Insider Sentiment ETF (NYSEMKT:NFO) could save you a lot of trouble. This exchange-traded fund focuses on stocks that have seen recent insider buying and rising earnings estimates among Wall Street analysts.

Why this ETF, and why insider buying?
Many folks get alarmed at reports of insiders selling shares of a stock. That's not necessarily bad news, though, as they might just need to generate money. (A flood of insider selling, however, is ominous.) On the other hand, insider buying is generally auspicious, as it often reflects confidence on the part of someone close to the company.

This is an ETF with an unusual angle, factoring insider buying into its holdings. It sports an expense ratio, or annual fee, of 0.65%, which is well below the typical mutual fund's fees. It has outperformed the S&P 500 over the past five years but lagged it over the past three. The ETF is on the small side, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

A closer look at some components
On your own you might not have selected Celgene Corporation (NASDAQ:CELG) or Ubiquiti Networks Inc. (NYSE:UI) for your portfolio, but this ETF recently counted them among its 100 holdings.

Credit: Victor, Flickr.

Celgene Corporation
Celgene has averaged annual growth of 32% over the past 20 years. And yet its stock is still attractive at recent levels with a forward P/E ratio of 17, well below its five-year average of 33.

Much of Celgene's success owes to its anemia drug, Revlimid, which is expected to generate about $5 billion in sales this year and potentially $11 billion by 2021. The company is looking to get the drug approved for other applications, too. But wait -- there's more! Celgene has other drugs selling well. Its new multiple myeloma therapy, Pomalyst, enjoyed sales of $305 million last year and $136 million in the recent first quarter. Its Abraxane drug, treating metastatic breast cancer, pancreatic cancer, and non-small-cell lung cancer, saw its sales grow by about 50% in the first quarter, year over year, and it's expected to generate close to $900 million this year.

Then there's Otezla, also known as apremilast, which was recently approved to treat psoriatic arthritis and might soon be approved to treat plaque psoriasis, a condition affecting even more people (some 3% of the population). The drug is expected to ultimately generate more than $1 billion. (It recently offered disappointing late-stage results treating a form of spinal arthritis, but that was only a minor setback, as other studies remain underway.)

Meanwhile, Celgene is investing heavily in other companies with promising early-stage drugs, a move that eventually could pay off handsomely. As my colleague Stephen Simpson has noted, Celgene has "a deep early stage pipeline of oncology drugs" and "meaningful label expansion opportunities for approved drugs."

Celgene offers no dividend, but it has been rewarding shareholders via share repurchases -- to the tune of roughly 9% of shares retired in just a few years. With more than $5 billion in cash and free cash flow topping $2 billion annually, it's well positioned to keep making smart investments in and purchases of other promising companies.

Ubiquiti Networks
Ubiquiti Networks offers high-performance networking technology for service providers and enterprises. It also offers no dividend and is planning to reward shareholders via share buybacks instead, having recently approved a $75 million repurchase. Many investors are apparently unimpressed, though, as the stock recently had nearly 9% of its outstanding shares sold short. They were unimpressed with the company's last quarterly earnings report, too, sending shares down more than 20%.

Still, you may want to consider it for your portfolio. That last disappointing quarter did feature revenue up 78% year over year and net income more than doubling. The company's net margin recently rose from 25% to a hefty 30%, and it's generating positive free cash flow.

The company is poised to benefit from a new FCC plan to spend $2 billion bringing Wi-Fi access to U.S. public schools and libraries. Its foreign growth potential is solid, too, as it generates less than a fifth of its revenue in the U.S. and is targeting many emerging markets, which can grow much faster than the U.S.

With a recent P/E ratio near 23 and a forward P/E near 17, the stock is attractively priced, given its growth rates and potential.