What: bluebird bio (NASDAQ:BLUE) is down over 20% today after abstracts for the American Society of Hematology (ASH) meeting were released this morning.

So what: ASH is one of the biggest medical conferences of the year for blood-based diseases. Investors had bid up shares of bluebird going into the data dump, so any less-than-positive data was sure to send shares down.

Unfortunately, that's what investors got -- at least in part.

The results are from two trials testing patients with beta-thalassemia major, a disease that causes a reduction in the production of hemoglobin, requiring frequent blood transfusions. Beta-thalassemia comes in two varieties -- major and minor -- and, within the major class, there are varying degrees of hemoglobin function depending on the patient's genes. The clinical trials are testing bluebird bio's LentiGlobin BB305, a gene therapy that adds a functioning copy of the hemoglobin gene to the patient's cells, in a variety of genotypes.

Patients with non-β00 and β0E genotypes, the less severe types of beta-thalassemia major, appear to be cured with LentiGlobin BB305. Every patient that has been treated remains transfusion-free.

Patients with the β00 genotype, who have no hemoglobin production, haven't been as fortunate. Two patients with the genotype have received a single transfusion since getting the LentiGlobin BB305 treatment. Another patient with the genotype hasn't been able to stop receiving transfusions post treatment.

Now what: All of this data from the abstracts is as of July 31, so the data presented at ASH next month will contain more up to date information. Longer follow up times could bring good news for the less severe genotypes since the longer they remain transfusion-free the more likely the treatment is truly a cure. For the β00 genotype, the situation probably isn't going to get better, and LentiGlobin BB305 may not be a cure. Depending on how much the treatment improves patients' lives by reducing the number of transfusions, LentiGlobin BB305 could still be a viable treatment for the β00 genotype.

Bluebird bio's investors have another reason to pay attention at ASH: data will be presented from a trial testing LentiGlobin BB305 in patients with sickle cell disease, a much larger market. The abstracts contained limited data for patients with sickle cell with just an update on one patient that remains transfusion-free.