blindness Spark Therapeutics Inc’s experimental gene therapy for a rare, genetic form of blindness has been proven effective, though it is unclear whether the benefit lasts over time, according to a preliminary review by the U.S. Food and Drug Administration. 

The therapy, Luxturna (voretigene neparvovec), would be the first ever gene therapy for any inherited disease to be approved in the U.S. 

In the company’s key study, 21 patients with retinal dystrophy caused by biallelic mutations in the RPE65 gene, were treated, with 10 in the placebo group. Of the 21 treated, 11 had significant improvement in light levels in both eyes. Fifteen of the patients scored a significant multi-luminance mobility testing (MLMT) improvement in the first eye treated. 

The treatment will be reviewed by an outside panel this week, which will decide whether the treatment should be approved or not. 

The FDA review states, “There is no available long-term follow up data to address whether the effect decays over time. Therefore, the duration of AAV2-mediated trans-gene expression leading to sustained clinical benefits beyond one year is unclear.” 

Although the hearing is Thursday this week, a final decision will be made by Jan. 18, 2017. The recommendation for or against on Thursday is not binding, but the FDA definitely takes it into consideration.