The first treatment was developed by Vertex Pharmaceuticals and Crispr Therapeutics. Casgevy will be sold in the USA for $2.2 million. But this amount must be set against the current cost of care, estimated at between $4 and $6 million over the lifetime of patients, according to the treatment's promoters. Bluebird Bio, the second laboratory whose treatment has been approved, quoted a price of $3.1 million for its Lyfgenia. The price difference led to a 40% plunge in BlueBird shares on Friday.

"These treatments represent a major advance in the field of gene therapy for patients with sickle cell disease, a rare and debilitating blood disorder that affects around 100,000 people in the US," explained the FDA's Peter Marks at a conference on Friday. The development of Crispr molecular scissors won the Nobel Prize in Chemistry in 2020 for Frenchwoman Emmanuelle Charpentier and American Jennifer Doudna.

Molecular scissors

CRISPR, which stands for Clustered Regularly Interspaced Short Palindromic Repeats, is an innovative gene-editing technology. It was discovered in the immune system of bacteria, where it is used to combat viruses by cutting their DNA.

CRISPR technology uses an enzyme called Cas9, which acts like a pair of "molecular scissors". This enzyme is guided by an RNA molecule, which is programmed to target a specific segment of DNA in a genome. When Cas9 reaches the target DNA segment, it cuts it, thus deactivating the target gene. What's more, once the DNA has been cut, researchers can also use CRISPR technology to insert new DNA segments at the site of the cut, enabling a defective gene to be replaced by a healthy version.