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.

The major indexes are all moving slightly lower this afternoon. As of 12:45 p.m. EDT, the Dow Jones Industrial Average (DJINDICES:^DJI) is down 45 points, or 0.26%, while the S&P 500 is off by 0.3% and the Nadaq is lower by 0.5%.

Big news on the day is the U.S. Senate's widely anticipated approval of Janet Yellen as the next Federal Reserve chair, along with some slightly disappointing data from the Institute of Supply Management.

Economists had expected a reading of 54.5 for the ISM nonmanufacturing PMI report for December, but the actual number was 53. While a figure above 50 still indicates expansion, the latest number doesn't represent as much as most were looking for.

Just because the major indexes are falling doesn't mean everyone is having a bad day. Shares of Sirius XM (NASDAQ:SIRI) are higher by 7% after Liberty Media (NASDAQ:FWONA) offered to buy the remaining shares of the satellite radio company that it already doesn't own. The offer price was the equivalent of $3.68 per share, but current Sirius shareholders will get Liberty Media shares. Sirius public shareholders would then own roughly 39% of Liberty's shares, because the company plans to perform a 2:1 split prior to the Sirius deal closing. Liberty's Class A shares are lower by 2.7% at this time.  

Another partnership making news this afternoon is the deal between (NASDAQ:BKNG) and General Motors Chevrolet. The deal would enable Chevrolet customers who have the OnStar service to access a customer service representative or, later in 2014, an app that can help with a Priceline booking on the go. Priceline says it is trying to capitalize on the fact that hotels are one of the most requested categories on the OnStar service by offering Chevrolet customers a better experience and cheaper hotel rooms than before. Shares of Priceline are down 0.1%, while GM is up more than 1.4%.  

Lastly, shares of Twitter (NYSE:TWTR) are down more than 5% after Morgan Stanley downgraded the stock this morning. The previous equal-weight rating was cut to underweight, while the price target of $33 per share was maintained. The firm believes that the risk/reward profile of the stock is nowhere close to being attractive at this time. The analyst feels that there is no guarantee that the company will ever become successful as the advertising market becomes increasingly crowded every day. Additionally, as it sits today, the stock has priced in a tripling of the company's socially enabled advertising market within the next three years, a goal that is going to be difficult to reach.  

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