Warren Buffett 's Berkshire Hathaway Inc. is believed to have seen a slump in returns because of the 7.09% stake in International Business Machines Corp. , which saw its stock price plummet throughout the last 30 days. Considering that the American Express Company represents approximately 14.68% of the same portfolio and the company gained little to 9% this month, one might infer that Warren Buffett won't see his profits drop below zero.