Scientists in Japan achieved a pioneering milestone by using CRISPR-Cas9 to selectively excise the third copy of chromosome 21 in human cells cultured in vitro, effectively removing the genetic cause of Down syndrome. This first-of-its-kind allele-specific editing suppresses DNA repair mechanisms temporarily to increase efficacy. Post-editing, cells exhibited normalized growth and altered gene expression favoring brain development and reduced inflammation and metabolic gene activity. While the study opens therapeutic possibilities, it raises critical ethical and clinical considerations.