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What happened

ImmunoGen (IMGN) is up 19.5% at 12:48 p.m. EST for no obvious reason other than it's a biotech that day traders like to play with.

So what

Rather than being valued on fundamentals, ImmunoGen is trading on sentiment. And day traders seem to have a positive sentiment about the biotech -- at least today.

Last week, the biotech pointed out that data from the phase 1 trial testing KL in patients with ovarian cancer were published in the Journal of Clinical Oncology, but that really shouldn't change the valuation much since most of the data had already been previously disclosed.

ImmunoGen used that data to design its currently underway phase 3 clinical trial, FORWARD I, which will ultimately define the biotech's valuation. Moves like today's will have little effect on ImmunoGen's valuation after it releases the phase 3 results in a few years.

Now what

Before the phase 3 trial results are announced, investors will get more data on mirvetuximab soravtansine that could actually justify a valuation change. Starting in the middle of this year, ImmunoGen plans to release data from FORWARD II, a phase 1b/2 trial testing mirvetuximab soravtansine in different combinations with other cancer drugs. If the combinations look like they're working, investors will have more confidence that FORWARD I, which is testing the drug as a monotherapy, will be successful, raising the stock price.

Until then, keep in mind that the sentiment of day traders can change on a dime, and long-term shareholders should be prepared to ignore news-less moves in both directions.