In vivo vs. ex vivo

The two ways to deliver a gene therapy: edit cells outside the body and return them (ex vivo), or edit them in place (in vivo).

• Ex vivo: cells, often blood stem cells or immune cells, are removed, edited and quality-checked in a lab, then infused back.
• That allows checking before dosing, but blood-stem-cell treatments require chemotherapy to clear bone marrow, and each costs around $2–3 million.
• In vivo: the editor is injected, usually in lipid nanoparticles or engineered viruses, and edits cells where they are; the liver is easiest to reach.
• It could be a single injection without chemotherapy, but targeting is harder to control and edits can't be checked before they happen.