CRISPR-Cas9 is a gene-editing system adapted from a bacterial immune defense that lets scientists cut and rewrite DNA at precise locations using a programmable guide RNA and the Cas9 “molecular scissors.” Once Cas9 makes a break in the DNA, the cell’s own repair machinery either stitches it back together in an error-prone way (knocking genes out) or uses a supplied template to make precise changes. On top of classic Cas9, newer tools like base editors and prime editors can rewrite single letters or small stretches of DNA without making full double-strand breaks.

This toolkit has completely changed biology: it’s now routine to switch genes on or off, build disease models, engineer crops and livestock, and run massive genetic screens. In medicine, CRISPR has moved from the lab to the clinic, with a focus on somatic (non-heritable) edits delivered either ex vivo (cells edited outside the body and reinfused) or in vivo (directly into patients, often via lipid nanoparticles or viral vectors). Safety issues—off-target edits, complex DNA rearrangements, immune reactions, and delivery-related toxicity—are active areas of research and regulation.

Headline CRISPR treatments so far:

• Casgevy (exa-cel) – first approved CRISPR therapy (US, UK, others) for sickle cell disease and β-thalassemia, by editing blood stem cells ex vivo to turn back on fetal hemoglobin.
• NTLA-2001 – in vivo CRISPR knock-out of the TTR gene for transthyretin amyloidosis; showed huge TTR reductions but is now under FDA clinical hold after a serious liver toxicity case.
• EDIT-101 – subretinal CRISPR for an inherited blindness (LCA10), with many participants showing meaningful vision improvements.
• Custom CPS1 base-editing therapy – a one-off CRISPR base-editing treatment designed for a single baby with a lethal metabolic disorder.
• Cardiovascular base editing (e.g., VERVE-101, ANGPTL3 programs) – one-time in vivo edits in the liver to permanently lower LDL and triglycerides.
• CRISPR-engineered immune cells – T cells with PD-1 knocked out or multiplex-edited CAR-T cells for cancer immunotherapy.

At the same time, the controversial case of gene-edited babies (germline CCR5 edits) has led to strong global consensus that heritable embryo editing is off-limits for now, while carefully regulated somatic CRISPR therapies continue to expand.