Geron has not disclosed the nature of these imetelstat responses, which is why the reveal of the ASH abstract on Thursday is such a big deal.

Investors want to see partial remissions (PRs) in the imetelsat abstract. Complete remissions (CRs) would be amazing but perhaps less likely because the drug's mechanism of action isn't well suited to reducing spleen size.

So, look for PRs in the ASH abstract. The more PRs, the better, and the more likely Geron's stock rally will continue. Just note that at Monday's close of $4.01, Geron is already worth more than $500 million, raising the risk of a stock sell off regardless of how good the imetelstat data look. Compounded by the high likelihood Geron raises money to fund future imetelstat studies.

But Geron shares will almost certainly fall -- and far -- if imetelstat only manages to produce clinical improvements (CIs) in the Tefferi study. This outcome won't differentiate imetelstat enough from Jakafi to matter much.

For those interested in how CIs, PRs and CRs are measured in myelofibrosis studies, here's an outline, courtesy of Geron:

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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