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 S&P 500 Index (SNPINDEX:^GSPC) continued to roar higher today, posting its fourth gain in five days as six out of 10 stocks ended in the green. The benchmark index is up nearly 30% in 2013; steady corporate earnings growth, the avoidance of the debt ceiling, and continued quantitative easing from the Federal Reserve have sent stocks higher this year, despite those who say the market is overvalued. The S&P 500 added seven points on Friday, or 0.4%, to end at 1,798. 

Regeneron Pharmaceuticals (NASDAQ:REGN) was one of the day's most notable decliners, shedding 3.1% to ensure its position on this notorious list for a second day this week. Regeneron also found itself toward the bottom of the S&P on Wednesday after reports surfaced that two prominent medical organizations touted the use of statins in treating cholesterol, as opposed to PCSK9 inhibitors. Regeneron's pipeline has one of the latter drugs in the works, and investors weren't pleased to see a competitive method of treatment getting an endorsement.

Data storage company Seagate Technology (NASDAQ:STX) saw shares slip 2.3%. The stock is close to 52-week highs at current levels, despite trading at just 10 times earnings and paying a 3.6% dividend. The tech sector, after ending as the only sector in the red yesterday, again underperformed on Friday as Wall Street reacts with caution to a quarterly report from Cisco Systems that severely disappointed investors. Fairly or not, Cisco is sometimes seen as a proxy for tech in general, bringing other companies in the sector up -- or down -- with it.

Finally, Express Scripts (NASDAQ:ESRX), which offers pharmacy benefit management plans to consumers, fell 1.7%. Some companies like UnitedHealth Group have started in-sourcing their pharmacy benefit management plans, or PBMs, cutting Express Scripts out and diminishing its business. This trend is certainly a dynamic to pay attention to as a shareholder, because if it proves economical for companies like UnitedHealth, then others may follow suit, seriously damaging Express Scripts' business.