The Masters of the Universe got hit hard by investors on Monday. Like $150 billion hard.

Shares of the top tech giants fell sharply on Monday after reports that U.S. antitrust regulators had divided up oversight of the sector, with the Department of Justice assuming responsibility for Alphabet and Apple and the Federal Trade Commission taking on Facebook and Amazon. This triggered fears that the government could mount challenges to the business models of the companies.

Shares of Alphabet dived 6.1 percent on Monday after the Wall Street Journal reported that the Justice Department is in the early stage of preparing an antitrust probe of the company. Reuters reported that the Department of Justice is also looking into Apple’s business for possible antitrust vi0lations. Shares fell 1.0 percent.

Facebook and Amazon will reportedly be overseen by the Federal Trade Commission. Facebook shares fell 7.5 percent. Amazon shares tumbled 4.6 percent.

All told, the decline wiped out around $150 billion in the market value of the four companies.

Twitter, which is much smaller than the giants and was not featured in the reports of antitrust probes, saw its shares fall 5.8 percent.

Any antitrust probes are in their early stages, according to reports. Any actual actions by antitrust authorities may be months or even years in the future. But the division of labor between Justice and the FTC brings the threat of action a little closer to reality and indicates that the companies cannot count on government complacency in the future.