Shares of Berkshire Hathaway ( BRK.A 0.06%) ( BRK.B 0.18%) are falling on Monday. The company's stock lost 4.3% as of 11:45 a.m. ET today and as much as 6.8% earlier in the day. The leg down comes as the S&P 500 lost 0.5% and the Nasdaq Composite lost 0.7% on the day.

"You're losing one of the most esteemed investors one could argue of all time," a source said of investors' reaction to Buffett's plan to step down.

With a market cap of $1.1 trillion, Berkshire Hathaway Inc. (BRK-B) engages in the insurance, freight rail transportation, and utility businesses worldwide. Headquartered in Omaha, Nebraska, the company provides property,