Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

New quantitative easing tapering concerns have the markets rocketing down in the afternoon trading session, with the Dow Jones Industrial Average (DJINDICES:^DJI) falling 126 points as of 2:30 p.m. EST. Most of the index's blue-chip member stocks have also nosedived on the day. Don't tell that to Merck (NYSE:MRK), however, as the Big Pharma stock has exploded for huge gains on the day. On the other side, Disney (NYSE:DIS) is among the Dow's leading laggards to begin the week. Let's check out what you need to know.

Merck's welcome surprise
Merck investors have watched the company deal with tough patent expiration-related sales declines recently, but they received a welcome surprise on Monday. Merck began its FDA submission for melanoma and cancer therapy MK-3475 today and projects to complete the application sometime in the first half of the year. It's an early filing for the company, and considering MK-3475's potential it's one of the biggest signs of optimism in Merck's future. The stock has surged by a whopping 6.6% today alone on the news.

Why's MK-3475 so big? In treating melanoma alone, it's not: Analysts project that the developmental drug could see about $500 million inpeak sales through melanoma therapy. However, Merck's looking to expand MK-3475's treatment indications around other types of cancer as well, with a total of 10 trials under way for different treatment options. In all, this could boost MK-3475's potential peak sales to $3 billion or even more, making the drug Merck's pipeline shining star and a huge hit if it can pass by regulators for multiple indications.

That's good news for Merck, which has seen its sales plummet recently due to the patent cliff. Former top seller Singulair has lost nearly 75% of its revenue over the past nine months. MK-3475 won't make up for such losses right away, but in the long term it's an almost certain blockbuster that should form the core of Merck's drug portfolio in the latter half of the decade.

Meanwhile, Disney's stock has sunk by about 2% today. That loss has come despite Disney's extraordinary success in its most recent animated film Frozen, which emerged as one of 2013's top film hits and remains strong in the new year. Frozen has racked up more than $712 million worldwide during its theatrical run and still managed to lead the international box office this past weekend despite losing the top spot domestically. Considering that Frozen has yet to open in China, Disney's set to cash in on much more than just the stellar take it's made so far from this lucrative property.

Fool contributor Dan Carroll has no position in any stocks mentioned. The Motley Fool recommends Walt Disney. The Motley Fool owns shares of Walt Disney. 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.