What happened

Shares of Atara Biotherapeutics (ATRA -2.61%), a leader in the off-the-shelf adoptive cell therapy space, lost as much as 22% of their value in early morning trading today. What event triggered this hefty sell-off?

Ahead of the opening bell, Atara dropped its first-quarter earnings report. The issue that seems to have drawn the ire of investors is the news that enrollment in the company's dual late-stage trials for tab-cel -- an off-the-shelf cell therapy for patients with Epstein-Barr virus associated post-transplant lymphoproliferative disease -- is progressing slower than the company originally anticipated. As of 2:19 p.m. EDT on Thursday, Atara's shares were still down by a lofty 19.1% in the wake of this first-quarter earnings report.

Green chalkboard chart showing a positive trend abruptly turn negative.

Image source: Getty Images.

So what

This clinical update shouldn't be taken as a surefire sign that tab-cel's late-stage trials are going off the rails by any stretch of the imagination. Fairly or unfairly, though, investors do tend to keep close watch on enrollment rates for experimental therapies for a variety of reasons.

Not only does enrollment rate help to clue in investors as to when a study might yield top-line results, but it can also provide valuable insight into the enthusiasm among patients and clinicians toward a particular therapy. That said, investors definitely shouldn't read too deeply into tab-cel's slow enrollment figures. It could boil down to nothing more than a logistical issue. 

Now what

After Atara's CEO Isaac Ciechanover abruptly decided to step down earlier this year, the company's shareholders have been on edge. So it's not surprising that investors are treating this enrollment news as a seriously bad omen.

The key issue for investors to keep in mind, though, is that Atara's underlying value proposition hasn't changed at all since the start of the year. All that matters is tab-cel's forthcoming data readouts. The rest is simply fodder for day traders and short-sellers.