Assistant Secretary of Labor for Employee Benefits Security Phyllis Borzi said Wednesday regulations or guidance may be needed to protect participants in 401(k) plans that allow investing in virtually all a brokerage has to offer.

Most 401(k) plans restrict investment choices to a limited selection of mutual funds offered by the plan provider. But some 401(k) plans offer a “brokerage window” that allows participants to buy stocks, bonds, mutual funds or ETFs as people do who have self-directed IRA, trust or taxable accounts.

Borzi’s statement came in the opening of a Department of Labor effort to find more information about these windows.

In a request for information from the public, the Labor Department acknowledged the windows may allow investors to create better customized, more diverse portfolios and take advantage of special asset classes unavailable in an often confusing multitude of plan investment alternatives.

However, the agency said opponents argue brokerage investments may be too costly and unsuitable for many 401(k) participants because they are not reviewed by plan fiduciaries.