Four stocks are responsible for almost all the Dow's gains in 2017

Trading Nation: Apple surges to 2-year high

The Dow Jones industrial average is off to a solid start in 2017 thanks in large part to just four of its 30 components.

Shares of Apple, IBM, Boeing and Visa accounted for 98 percent of the Dow's gains year to date. Apple's stock alone accounted for nearly a third of the index's gains.

"With Apple and IBM, you have to figure they got too cheap," said Maris Ogg, president at Tower Bridge Advisors. "When you have a market this expensive, people go out looking for bargains."

The Dow outperformed the and Nasdaq composite in 2016, lifted by a massive post-U.S. election rally. Shares of Goldman Sachs, UnitedHealth Group, IBM and 3M provided the largest points contributions on the Dow last year. They accounted for 46 percent of the Dow's 2016 gains.

The Dow is up around 1.6 percent in 2017, having hit a record high earlier in the session.

— CNBC's Chris Hayes contributed to this report.