A decent pace of existing home sales for February and another pullback for the U.S. dollar was enough to get the market started on a bullish foot this week. It just wasn’t enough to keep it in the black. Thanks to a last minute pullback, the S&P 500 finished the day at 2104.42, down 0.17%.

Vertex Pharmaceuticals Incorporated (NASDAQ:VRTX), Kansas City Southern (NYSE:KSU) and ISIS Pharmaceuticals, Inc. (NASDAQ:ISIS) were among the biggest contributors to that marketwide weakness. Here’s what happened.

ISIS Pharmaceuticals (ISIS)

The news from ISIS Pharmaceuticals was technically good, but perhaps already baked into the price of ISIS shares… and then some.

Here’s what you’d expect to be the good news: the Phase 1 trial of antisense drug ANGPTL3 is producing encouraging results. ISIS Pharmaceuticals Vice President Richard Geary specifically noted:

“We are encouraged with the performance of ISIS-ANGPTL3Rx in healthy volunteers.  Based on data from preclinical models of hyperlipidemia, we expect to see even greater lipid reductions in patients with hyperlipidemia than in healthy volunteers. In fact, in this study we observed that healthy volunteers with higher baseline lipid levels experienced larger lipid reductions than those with lower baseline lipid levels.”

The bad news is, given the stock’s 150% gain since July of last year, there simply may not have been any room left for ISIS stock to move higher. A “buy the rumor, sell the news” mindset sent ISIS lower by nearly 6% today.

Kansas City Southern (KSU)

Shares of railroad stock Kansas City Southern fell a hefty 8% on Monday after the company announced its top line for 2015 wouldn’t be as a strong as first presumed. Specifically, KSU said its revenue growth for the current year would only be in the low single digits, with fiscal first quarter revenue likely to kick off the soft patch with a 4% year-over-year decline.

The bulk of the slowdown is attributable to soft growth in energy demand as well as an exceedingly strong U.S. dollar. Non-energy-related commodities performed perfectly in line with expectations, but it wasn’t enough to keep KSU shares afloat.

Though it didn’t lower its guidance, shares of rail-transportation competitor CSX Corporation (NYSE:CSX) were off more than 4% on Monday as well; investors are assuming the headwinds Kansas City Southern is facing apply universally. Railroad peer Union Pacific Corporation (NYSE:UNP) also fell sharply, closing down nearly 4%.

Vertex Pharmaceuticals (VRTX)

A cystic fibrosis treatment regimen Vertex Pharmaceuticals had been working on doesn’t work as well as many investors were hoping.

The drug in question, simply referred to as VX-661 for the time being, was actually being tested by Vertex Pharmaceuticals in conjunction with its existing (and marketed) cystic fibrosis drug Kalydeco. While the Phase 2 test indicated a solid response after four weeks of therapy, the more critical twelve-week efficacy results fell short of expectations even though the benefit was deemed “statistically significant.”

Supporters were quick to point out that VRTX remained attractive, as Kalydeco by itself was still an attractive cystic fibrosis treatment option. But, when all was said and done, the VRTX stock price closed at $125.79 on Monday, down 4% for the session.

As of this writing, James Brumley did not hold a position in any of the aforementioned securities.