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 ImmunoGen (NASDAQ:IMGN) slid almost 17% today, after its earnings report showed a larger-than-expected loss. An underwhelming licensing agreement with Roche (PINK: RHHBY.PK) for a potential breast cancer drug, also contributed to the decline.

So what: ImmunoGen's earnings came in at $4.1 million, but losses of $0.30 per share were $0.06 worse than the analyst consensus. Full-year forecasts still project the same $78 million to $82 million loss that the company originally offered in August.

ImmunoGen and Roche's combination development-stage breast cancer drug, T-DM1, was originally thought to be licensed to Roche at a 5% sales royalty rate. A disclosure filed today reported that the rate begins at 3%, only increasing to 5% if the drug generates over $700 million per year in sales. That prompted analysts to significantly lower their royalty estimates. Joel Sendek of Stifel Nicoulas dropped his estimated range from $2.4 million next year, and up to $176 million in 2018, to $300,000 and $126 million in the same time frame.

Now what: After the drop, ImmunoGen now trades at a new 52-week low, and its market cap has shrunk to less than $1 billion. However, ImmunoGen has four other major development-stage cancer drugs, as well as a few other compounds in various stages of development. A $50 million reduction in high-end potential revenue is, of course, a significant drop in T-DM1's value, so investors will now have to consider whether the company has any other compounds or partnerships in its pipeline that might make up for it.

Want more news and updates? Add ImmunoGen to your Watchlist now.

Fool contributor Alex Planes holds no financial position in any company mentioned here. Add him on Google+ or follow him on Twitter @TMFBiggles for more news and insights.

Motley Fool newsletter services have recommended buying shares of ImmunoGen. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.