The FDA has slapped a clin­i­cal hold on the IND filed by Ver­tex $VRTX and CRISPR Ther­a­peu­tics $CR­SP on a new gene ther­a­py for sick­le cell dis­ease.

The two biotechs an­nounced an al­liance on sick­le cell dis­ease late last year, co-de­vel­op­ing CTX001, which was made with CRISPR/Cas9 tech. The drug is de­signed to pro­duce high lev­els of fe­tal he­mo­glo­bin (HbF; he­mo­glo­bin F) in red blood cells.

In their state­ment, the com­pa­nies ne­glect­ed to pro­vide much de­tail, say­ing on­ly that the hold was put in place “pend­ing the res­o­lu­tion of cer­tain ques­tions that will be pro­vid­ed by the FDA as part of its re­view of the IND.”

At the same time, re­searchers for the part­ners plan to pro­ceed with a Eu­ro­pean study. CRISPR, which had been buoyed in re­cent days, saw its shares plunge 20% on the news.

The two com­pa­nies’ col­lab­o­ra­tion us­ing CRISPR/Cas9 gene edit­ing tools dates back to 2015, when Ver­tex hand­ed over $105 mil­lion to get the pact start­ed, with an ini­tial fo­cus on cys­tic fi­bro­sis. He­mo­glo­binopathies like sick­le cell dis­ease were a sec­ondary spot­light in the deal.

Gene ther­a­pies have been gain­ing steady trac­tion in the US, with Spark Ther­a­peu­tics pick­ing up a pi­o­neer­ing FDA OK for Lux­tur­na last year. The FDA, though, re­cent­ly put a gene ther­a­py pro­gram at Sol­id Bio­sciences on hold af­ter Penn’s James Wil­son abrupt­ly re­signed from their sci­en­tif­ic ad­vi­so­ry group, warn­ing about the risks as­so­ci­at­ed with “high sys­temic dos­ing of AAV [ade­no-as­so­ci­at­ed virus].”

We’ll keep you post­ed on the rea­sons for this lat­est hold as we learn more about it.