The CRISPR-Cas9 genome editing system has inarguably revolutionized medical research, promising a future where scientists can treat a vast array of human illnesses from cancer to blindness. But since its discovery in 2012, progress towards human trials has been slow. Now, another setback has appeared with the FDA pausing one of the first human trials using the technique in the United States before it even begins.

The proposed treatment to be trialed is called CTX001, and in the US it was set to be investigated for patients suffering from sickle cell disease. The treatment isolates stem cells from a patient's blood and then uses CRISPR to make a single genetic change designed to raise levels of fetal hemoglobin in red blood cells.

The Phase 1/2 human clinical trial in the US was set to commence later this year until a recent announcement from CRISPR Therapeutics, the company developing the treatment, revealed the US Food and Drug Administration (FDA) has just placed a clinical hold on the trial. At this stage the only details released by the company state that the hold on the trial is pending "the resolution of certain questions that will be provided by the FDA as part of its review of the IND [Investigational New Drug Application]."

No information has been revealed clarifying exactly what the FDA's enquiries are, but upon announcement of the clinical trial hold CRISPR Therapeutics stocks tumbled by nearly 19 percent.

The US trial was set to run in conjunction with a European trial targeting a condition called beta-thalassemia using the same treatment. The status of the European trial is reportedly unaffected by this FDA decision and still on track to be initiated later in 2018, making it the first CRISPR human trial to take place in Europe.

The sickle cell CTX001 trial was set to be one of the first two CRISPR trials to commence in the US in 2018. The other trial, being run out of the University of Pennsylvania, targets three types of cancer: multiple myeloma, sarcoma, and melanoma. It is still reportedly on track for commencement very soon, although its clinical trial status is still labeled as "recruiting."

This roadblock is yet another affirmation of the challenges scientists are facing in translating this momentous technological advance into safe and effective human treatments. The only current human trials using the CRISPR technique are reportedly underway in China, but there have been concerns that the country may be racing forward in its trials too quickly.

While the last few years have undeniably proved exciting as CRISPR has made sophisticated gene editing a reality, the next couple of years are set to show us just how easy or difficult it will be to translate revolutionary technology into safe human clinical treatments.