The US Food and Drug administration has delayed the start of a human study of a genetically engineered therapy for treating sickle cell disease. In May, the agency issued a clinical hold on the investigational new drug application filed by Vertex and CRISPR Therapeutics to test CTX001, a new autologous gene therapy made with CRISPR–Cas9 technology, pending resolution of questions from the agency. No other details on the reason for this hold were disclosed. CTX001 is designed to produce high levels of fetal hemoglobin (HbF) in red blood cells. HbF is a form of the oxygen-carrying hemoglobin that is naturally present at birth, and is then replaced by the adult form of hemoglobin. The hope is that elevating HbF using CTX001 can alleviate transfusion requirements and debilitating crises for sickle cell and β-thalassemia patients. In December 2017, CRISPR presented preclinical data showing greater than 90% editing of hematopoietic stem cells at the target site, leading to clinically relevant increases in HbF. According to CRISPR Therapeutics, which is headquartered in Zug, Switzerland, a European trial of CTX001 in patients with transfusion-dependent β-thalassemia remains on track to begin later this year. The company's collaborator Vertex has also exercised an option to co-develop CTX001 for hemoglobinopathies. In February 2018, the University of Pennsylvania in Philadelphia began enrolling patients in the first CRISPR trial in the US, to treat multiple myeloma, melanoma and sarcoma, using engineered autologous NY-ESO-1 T cells, edited to remove endogenous TCR and PD-1. Several trials of cells engineered using CRISPR–Cas9 technology are recruiting in China.