In a remarkable advancement in genetic medicine, researchers have successfully used the gene-editing tool CRISPR-Cas9 to eliminate the extra chromosome responsible for Down syndrome — at least in laboratory-grown human cells. This extra chromosome, chromosome 21, is what underlies the developmental and physical characteristics associated with the condition.

The study, conducted by a team led by Ryotaro Hashizume at Mie University in Japan and published in the journal PNAS Nexus in early 2025, reports that “allele-specific multiple chromosome cleavage” can achieve what the authors call “trisomy rescue” — that is, removal of the surplus chromosome in cells with trisomy 21.

In practice, the team used CRISPR-Cas9 to precisely recognize and cut only the extra copy of chromosome 21, while leaving the normal two copies intact (one from each parent).

After this targeted “cut,” the extra chromosome is lost during subsequent cell division. The researchers applied this method not only to pluripotent stem cells derived from individuals with Down syndrome, but also in skin fibroblasts — a type of fully differentiated, non-dividing cell.

What makes this result truly noteworthy is that after removal of the additional chromosome, the previously trisomic cells showed a normalization of their behavior: gene expression patterns and protein production more closely resembled those of normal (disomic) cells, cell proliferation improved, and antioxidant capacity and other cellular functions returned toward typical levels.

The authors of the study and independent experts alike emphasize that this remains a proof-of-concept — successful only in vitro (in culture dishes), not in living organisms.

Nevertheless, the significance of this milestone should not be understated. The work demonstrates — for the first time — that CRISPR can remove an entire extra human chromosome rather than editing or correcting individual genes. This could pave the way toward therapies that address the root genetic cause of Down syndrome, rather than merely managing symptoms.

Still, formidable challenges remain before such an approach could ever be used clinically — if indeed it ever will. For one, editing in cell culture is much simpler than editing in a living human being. It is not yet known how to deliver CRISPR safely and efficiently to all or most of the relevant cells in a developing fetus or a living person. There is also concern about “off-target effects” — the possibility that the CRISPR machinery could damage other chromosomes or DNA sequences by mistake.

Furthermore, even in this study, the conversion from trisomy to disomy was not 100% efficient — only a fraction of cells regained the normal chromosome count.

Finally, beyond technical and safety hurdles lies a host of ethical, social, and regulatory questions. Interventions that eliminate extra chromosomes at embryonic or fetal stages could raise deep questions about genetic “normalization,” disability rights, and diversity. Indeed, some ethicists warn that the prospect of erasing conditions like Down syndrome at the genetic level might affect how society perceives individuals living with those conditions.

In summary: this new research marks a historic proof-of-concept. Using CRISPR-Cas9, scientists have for the first time removed the extra chromosome 21 from human Down-syndrome cells, and observed a restoration of gene expression and cellular functions toward normal. But we are still a long way from any human therapy. Much more work is needed — both technically and ethically — before this approach could pave the way to preventing or curing Down syndrome in real patients.