There's no two ways about it -- it's been a phenomenal year for shares of Alexion Pharmaceuticals ( ALXN). Shares of the $20 billion pharmaceutical firm have rallied 50% since the first trading day of January, besting the broad market's performance by a wide margin. Alexion emerged from obscurity on the heels of its Soliris drug, a treatment for rare blood disorders. By focusing on niche treatments of rare diseases, Alexion gets the big benefits of orphan drug status, and it doesn't have to compete with big pharma rivals. >>10 Stocks of Top-Performing Mutual Funds in 2012 While Soliris is Alexion's sole drug, the firm has been working to gain its FDA approval for other autoimmune diseases. The result is more sales without more development costs associated with trying to bring other drugs to market. It's a perfect strategy for an orphan drug, which gets longer patent protection, and Soliris has already proven useful for other indications. That performance has helped the firm build a large cash backstop (more than $800 million at last count, offset by some added debt from its Enobia acquisition) that should help to fuel its future R&D costs. Let me be clear, though: ALXN isn't a cheap stock right now; it's actually quite overpriced. But momentum continues to be rock-solid in this stock, and buyers aren't scarce. That's why ALXN gets Rocket Stock status this week.