Mutual funds dominate the 401(k) landscape, but an investor can opt for exchange traded funds in their 401(k)s if the employer includes a brokerage window, or self-directed 401(k), option.

The brokerage window basically acts like a brokerage account embedded within the retirement plan, writes Adam Zoll for Morningstar. [Build a Diversified 401(k) Portfolio with Six ETFs]

Around 17% of 401(k) plans offered a brokerage window option as of 2012. Potential investors, though, should be aware that plans may vary, with some options only allowing for mutual funds while others include stocks and ETFs. [How Low-Cost ETFs Can Help Boost Your 401(k) Returns]

Before diving in head first, there are some considerations. Investors should look into a brokerage window if it makes sense. Many 401(k)s offer a diverse range of asset classes, but some investors may want more options, such as dedicated emerging market, high-yield, real estate or small-cap exposure to name a few. With ETFs, investors can track a wider range of options.

A 401(k) investors should not utilize the brokerage window as a means for trading or timing the market. The option is not an invitation to begin dabbling in gut instincts. A well-designed 401(k) provides the right mix of equity, fixed-income and alternative assets that suit an investor’s time horizon.