US stocks have taken another small step backwards after a plunge in the price of oil dragged down shares of energy producers.
The losses overshadowed gains for technology companies and other areas of the market.
The Standard & Poor's 500 index dipped by a fraction of a point, down 0.30 to 2629.27, and is down just 0.5% so far this week.
However, even those modest movements could count as notable in a year that has been unusually calm and easy for investors.
It was the fourth straight loss for the index, the first time that has happened since March.
The Dow Jones industrial average fell 39.73 points, or 0.2%, to 24140.91, the Nasdaq composite rose 14.16, or 0.2%, to 6776.38 and the Russell 2000 index of small-cap stocks lost 7.88, or 0.5%, to 1508.88.
Stocks have been mostly drifting lower this week following a strong run for markets this year.
The ups and downs have come as the Senate and House of Representatives try to iron out differences in their proposals to overhaul the tax system, and investors shift their portfolios towards companies that stand to benefit most from lower rates.
"It looks like we topped out last week and we've been rolling a bit," said Phil Orlando, chief equity market strategist at Federated Investors.
"The reality is we had a phenomenal run here, and we looked a little overbought in my mind. So I wouldn't at all discount a little bit of a correction here of 2 or 3%."
The market, which is still up more than 17% for the year, is also in a relatively quiet period, Mr Orlando said.
Companies have finished reporting how much profit they made in the summer, and fourth-quarter reports will not start again in earnest for more than a month.
That can lead to a drifting market.
The market's biggest movers were energy stocks, which sank with the price of oil.
Benchmark U.S. crude fell 1.66 dollars to settle at 55.96 dollars per barrel. Brent crude, the international standard, lost 1.64 dollars to 61.22 dollars a barrel.
That led to a 1.3% loss for energy stocks in the S&P 500, the sharpest drop among the 11 sectors that make up the index.
Companies in the dental industry were also weak, hurt by fears that their industry is the next that Amazon will upend.
Analysts at Morgan Stanley cut their financial estimates for the companies on signs that Amazon has gotten access to a key dental equipment maker and may line up others in coming years, among other factors.
On the winning side was DaVita, which jumped to the biggest gain in the S&P 500 after UnitedHealth Group said it will buy DaVita's medical group, which serves patients through nearly 300 medical clinics.
Technology stocks also rose, and they recovered some of their losses from earlier in the week.
In markets overseas, Asian stocks slumped. Japan's Nikkei 225 index lost 2% for its worst day since March. The Hang Seng in Hong Kong dropped 2.1%, and South Korea's Kospi lost 1.4%.
In Europe, markets trimmed their losses as the day progressed. Germany's DAX dropped 0.4%, and France's CAC 40 ended little changed. The FTSE 100 in London rose 0.3%.