On Jun. 30, 2025, as in something out of science fiction, a team of researchers reached into the genome of a seven-month-old baby with a base editing therapy targeted specifically to repair his unique mutation. Time was of the essence, as the rare, life-threatening genetic disorder caused by a mutation in his carbamoyl phosphate synthetase 1 (CPS1) gene impairs the body’s ability to process protein, causing a toxic build-up of ammonia in the blood, affecting brain function.

Half the babies with this urea cycle disorder die before their first birthday, and even when they survive, they experience severe developmental delays. The speed was remarkable: it took less than seven months to go from having the sequence in hand to delivering this first treatment of its kind, made just for him. Now nine month old, baby KJ, as he is known, appears healthy.

While baby KJ made news around the globe, the latest generations of gene editing medicines garnered fewer headlines but made equally impressive strides. In March, Beam Therapeutics released the results of a phase 1/2 trial of in vivo base editing medicine for α-1 antitrypsin deficiency. In April, Verve Therapeutics released dramatic cholesterol-lowering clinical results with their in vivo base editing treatment for familial hypercholesterolemia. And in May, Prime Medicine announced that the first patient to receive their prime editing medicine, an 18-year-old man with chronic granulomatosis disease, saw his health improve rapidly after a single injection of the therapy.

Not to be forgotten, the original CRISPR–Cas9 therapies also made some news. Intellia Therapeutics’ nexiguran ziclumeran (nex-z) — the first systemic gene editing therapy in humans — entered phase 3 trials. This treatment, for hereditary transthyretin amyloidosis, reduced serum transthyretin by 90% at 28 days in phase 1 studies.

Trials with traditional CRISPR therapies still outnumber base and prime editors, but the new kids on the block are gaining ground. As of June, at least 20 clinical trials with base or prime editors are underway, and 9 have released some results. The main distinction is in the enzymes: the newer base and prime editing medicines combine a CRISPR–Cas targeting agent with enzymes that modify or insert specific nucleotides into the DNA without creating double-stranded breaks. As such, these editors are more precise than CRISPR–Cas9 technologies and minimize the risk of mayhem caused by endogenous repair mechanisms recruited to fix double-stranded breaks.

CRISPR–Cas9-based medicines are likely to remain confined to what the tool does best: knock out unwanted or toxic genes. In contrast, base editing can be used to correct mutated genes by performing chemistry changes on single, targeted nucleotides.

Base editors use a modified Cas9 protein known as a ‘nickase’ that creates nicks or breaks in a single strand of DNA, combined with a deaminase that acts at that site to remove an amino group from a base, turning cytosine to uracil, or adenine to inosine. Prime editors can rewrite a segment of DNA and replace the original sequence by using a reverse transcriptase in collaboration with a cellular repair mechanism (search and replace editing).