Stocks are holding on to this morning's gains, with the S&P 500 (SNPINDEX:^GSPC) and the narrower, price-weighted Dow Jones Industrials Average (DJINDICES:^DJI) up 0.22% and 0.31%, respectively, as of 1 p.m. EST. Within the Dow, winners outnumber losers by more than 2-to-1 right now. Let's go over the largest percentage losers of the day so far.

I see no immediate news item that would push shares of payment card issuer American Express (NYSE:AXP) down today, so I'm going to take this opportunity to offer a "longer-horizon" comment. In his latest annual letter to shareholders, released last Friday, Berkshire Hathaway (NYSE:BRK-B) CEO Warren Buffett praised American Express as one of his "Big Four" investments. Noting that the company's share repurchases had lifted Berkshire's stake from 13% to 13.7%, he added:

The earnings that the four companies retain are often used for repurchases -- which enhance our share of future earnings -- and also for funding business opportunities that are usually advantageous. Over time we expect substantially greater earnings from these four investees.  

The bottom line for Mr. Buffett, which ought to be of great interest to Berkshire investors and value-oriented investors, is that "Berkshire's ownership interest in all four companies is likely to increase in the future." Let's be clear: That could be due to additional purchases by Berkshire or simply a result of continuing share buybacks by the company (or a combination of both). Either way, with American Express currently trading at 13.5 times the estimate for the next 12 months' earnings per share, it looks like Berkshire would receive adequate value for its dollar.

Shares of Home Depot (NYSE:HD) are also down today. Speaking of share buybacks, yesterday I pointed out that the home improvement retailer is one the top-five companies in terms of the size of its share buyback authorization as of the end of February. (In fact, it was No. 1, with a massive $17 billion authorization.) At a current valuation of nearly 20 times the estimate of the next 12 months' EPS, it's less clear, at first glance, that repurchases at current levels will create value equivalent to those of Amex's stock.

Finally, shares of energy supermajor ExxonMobil are also losing ground today. Again, I can see nothing specific affecting the company, but the energy sector is mixed today. The market is like that most of the time -- it's best not to try to find an explanation for all of its gyrations.

Fool contributor Alex Dumortier, CFA has no position in any stocks mentioned; you can follow him on LinkedIn. The Motley Fool recommends American Express, Berkshire Hathaway, and Home Depot. The Motley Fool owns shares of Berkshire Hathaway. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.