What Is CRISPR? The Gene-Editing Tool Borrowed From Bacteria

CRISPR stands for clustered regularly interspaced short palindromic repeats. The National Human Genome Research Institute describes it as a technology that research scientists use to "selectively modify the DNA of living organisms," adapted from naturally occurring genome editing systems found in bacteria.^[1] MedlinePlus similarly says CRISPR-Cas9 adapts a bacterial immune defense.^[2]

Guide and scissors

In plain terms, the system has two parts. Researchers build a small piece of RNA with a short guide sequence that attaches to a specific target sequence in DNA. The guide leads the Cas9 enzyme to that spot, and Cas9 cuts the DNA there. Afterward the cell's own repair machinery modifies or replaces the genetic segment.^[2] A medical journal review describes the same pair as Cas9, a programmable nuclease, and a single guide RNA, noting that the cut can inactivate a problem gene or allow a precise correction.^[4] NHGRI calls such tools molecular scissors that can remove, add or replace DNA where it was cut.^[3]

From lab bench to first approved therapy

Scientists use editing to study disease by changing genes in animals such as zebrafish and mice.^[3] In medicine, the U.S. Food and Drug Administration approved Casgevy on December 8, 2023, calling it the first FDA-approved treatment to use a novel genome editing technology. It edits a patient's own blood stem cells to raise production of fetal hemoglobin, which prevents red blood cells from sickling, and was approved for patients 12 and older with recurrent vaso-occlusive crises. In trials, 29 of 31 evaluable patients (93.5 percent) had no severe episodes for at least 12 consecutive months.^[5]

• Cells are removed, edited in the lab, and returned as a one-time treatment after chemotherapy conditioning.^[5]
• MedlinePlus says clinical trials also explore conditions such as cystic fibrosis and hemophilia, while scientists are still working to determine whether the approach is safe and effective in people.^[2]

Safety, ethics and rules

Most edits target somatic, or non-reproductive, cells and are not inherited. Changes to egg or sperm cells or to an embryo could be passed to future generations, and MedlinePlus states that germline and embryo editing are currently illegal in the United States and many other countries because of ethical and safety concerns.^[2] NHGRI frames it somewhat differently: most scientists oppose germline editing for now, the NIH does not fund human embryo editing research, and international frameworks restrict such uses until safety improves.^[3] It lists further questions, including consent for embryo treatment, fair access and cost, enhancement beyond disease prevention, and tools sometimes cutting at unintended locations.^[3] The journal review likewise raises germline ethics, equitable access and off-target effects needing long-term monitoring.^[4]

The sources describe CRISPR as a powerful, still-maturing tool whose rules are being shaped as the science advances.