Berkshire Hathaway B shares slid 4.1% Monday afternoon, after Warren Buffett stunned shareholders on Saturday with the announcement that he plans to step down as CEO at the end of the year.

Billionaire hedge fund manager Bill Ackman has been attempting to use real estate developer Howard Hughes Holdings (NYSE: HHH) to create a "modern-day Berkshire Hathaway" for some time. In January, Ackman proposed taking a majority stake in Howard Hughes by purchasing shares for $85.

Investors kickoff the week awaiting more developments on President Trump’s trade deals and the decision on interest rates by the Federal Reserve out Wednesday. Optimism is improving after the S&P 500 notched the longest winning streak in 20 years last week.