A team at Mie University in Japan has successfully used CRISPR-Cas9 technology to selectively remove the extra chromosome 21 responsible for Down syndrome in laboratory-grown human cells. This allele-specific editing targets only the trisomic chromosome, resulting in altered cellular behavior, normalized growth, and gene expression changes linked to brain development. This pioneering achievement opens new avenues for potential therapies, though it raises ethical questions surrounding genome editing of human embryos.