Earnings season can send stocks roaring up and down, but in the biotech sector, it's a little harder to stick by just the numbers. With so much emphasis placed on developing successful drugs that can hurdle the FDA regulatory blockade, there's more important fish to fry for most companies than just meeting a single quarter's projections.

Yet shares of Immunogen (IMGN) fell more than 2% on Friday anyway, cast down by earnings that disappointed Wall Street. Should you be worried by the bad numbers -- or is today just a day to forget?

Disappointment? Only to Wall Street
Still, best to review the numbers Immunogen released to see what kind of ground the company's standing on.

No one should really be surprised that the company reported a net loss, but the size of the loss shocked some investors. Immunogen's net loss doubled over this time last year to $24.4 million, or $0.29 per share. In 2011, the company lost just $12.8 million -- $0.17 per share -- for the quarter ending on Dec. 31.

Immunogen increased outstanding shares by 10% in the time between the two quarters, making the EPS loss less drastic than it could have been otherwise. Still, the numbers exceeded analyst expectations of a $0.25-per-share loss.

Revenue also dropped year over year -- from $7.62 million last year to $2.61 million this quarter -- although 2011's mark was boosted by a $5 million milestone payment. However, considering Immunogen doesn't have any approved drugs on the market yet, paying attention to the revenue decline misses what's ahead for the company.

The only thing to pay attention to
In fact, there's only one number (or at least a series of numbers and letters) Immunogen investors should really watch: T-DM1.

That's the name of the cancer-fighting therapy the company's developing alongside Roche (RHHBY 0.60%), and its fate will mirror Immunogen's. Roche expects the FDA to approve the drug early this year, followed up by European approval later in 2013. If it clears regulatory hurdles -- and given its success in reducing the risk of death in clinical trials, it should -- T-DM1 could be on its way to blockbuster status.

That'd make everyone on Wall Street quickly forget about Immunogen's disappointing quarter. It's important enough for the company to have its first drug reach the market, but T-DM1's role in treating HER2-positive breast cancer -- which represents between 15% and 20% of all breast cancers -- gives Immunogen a sizable market to work with. Analysts have pegged peak sales at anywhere from $2 billion to as high as $5 billion.

Of course, partner Roche could always make a move first. With such a blockbuster in hand, it wouldn't be surprising at all if Roche simply swallows up Immunogen to get its hands on a foundation for its pharmaceutical portfolio.

This disappointing quarter won't even dent shareholder glee if that comes to pass.

More important dates ahead
Immunogen might have had a poor quarter, but don't train your eyes on the sizable net loss or the stock's decline today. The real make-or-break point for this company will come with the FDA's decision on T-DM1, with a PDUFA date set for Feb. 26. Mark your calendars, Immunogen investors.