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

What: Shares of Alexion Pharmaceuticals (NASDAQ:ALXN), a biopharmaceutical company engaged in the development of therapies to treat rare and ultra-rare diseases, rocketed higher by more than 21% after reporting better-than-expected fourth quarter earnings results.

So what: For the quarter, Alexion reported a 38% increase in sales of Soliris, its only FDA-approved drug, to $441.9 million from $320.5 million in the year-ago period. Adjusted net income climbed 45% to $0.87. By comparison, Wall Street had forecasted just $0.83 in EPS on $430.7 million in revenue. Looking ahead, Alexion issued considerably better guidance for fiscal 2014 than Street estimates. It anticipates revenue of $2 billion to $2.02 billion versus estimates of $1.96 billion, and EPS of $3.70-$3.80 compared to the $3.42 estimate on the Street. The upside EPS surprise is primarily due to a lower expected tax rate of just 10%-11% in 2014.

Now what: What we have here is what I'd refer to as a mixed bag. On one hand, the continued growth for Soliris is impressive, and I certainly wouldn't deny Alexion or its bulls the right to tout that investing in ultra-rare disease drugs can be quite profitable. Soliris, because it has orphan-drug status and treats rare disorders, is also protected from generic competition for a long time to come. However, Alexion is also bloated from a fundamental perspective at a valuation of nearly $32 billion. In other words, Alexion is now valued at 16 times estimated 2014 full-year sales, and much of its pipeline still revolves around a single drug. As we've seen from drugmakers like Affymax last year, there can be a lot of inherent dangers of relying on one drug to drive an entire portfolio. With that being said, I'd suggest taking this gift of a rally on the expectation of lower taxes and rising Soliris sales in 2014 and head for the exits.