Eight in 10 sectors advanced in the stock market today, as the S&P 500 Index (SNPINDEX:^GSPC) added eight points, or 0.5%, to end at 1,857. The health-care sector again stood out as an underperformer, continuing a week-long trend that began last Friday. While most stocks rose today, Biogen Idec (NASDAQ:BIIB), Vertex Pharmaceuticals (NASDAQ:VRTX), and PG&E (NYSE:PCG) all finished at the bottom of the S&P.

Biogen Idec, a $70 billion biotech with products treating a range of ailments from hemophilia to autoimmune diseases, lost 5.1% Friday. The steep losses preceded good news from the FDA, which officially approved Biogen Idec's hemophilia B treatment Aprolix this afternoon. With only an estimated 25,000 people suffering from the disease worldwide, Aprolix is an "orphan drug," or, in other words, a drug that treats a very rare condition and costs customers a fortune. 

Vertex Pharmaceuticals, another biotech, shed 4.2% today. Investors are probably thankful the week's finally over; Vertex stock dropped 8.2% in the last five sessions alone. Vertex isn't yet profitable, but its cystic fibrosis drug, Kalydeco, could offer investors big returns. Drugmakers constantly try to grow the market for their products, oftentimes by means of making offerings applicable to a wider variety of indications, expanding the target market. The company is currently awaiting data on how patients with cystic fibrosis respond to Kalydeco and another Vertex drug, hoping the combo will prove more effective to a wider population. 

Aftermath of 2010 San Bruno explosion. Source: Flickr

Lastly, shares of PG&E lost 4% Friday. Notably the only non-biotech on today's list, PG&E is an electric and natural gas utility company. Utilities don't have a reputation for being wildly volatile, so you might expect some horrible news to be behind today's slump. You'd be right: The company said it expects the Feds to press charges against it for a September 2010 pipeline explosion in California that left eight dead. PG&E has set aside $2.7 billion to deal with the fallout from the incident, an expense that will almost certainly impede the $19 billion company's ability to grow.

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