[Targeted therapy for ultra-rare diseases].

Nano-rare diseases, affecting fewer than 30 individuals worldwide, are mostly genetic and severe, with no effective treatments available. Nucleic acid-based therapies, such as antisense oligonucleotides, allow for the targeted modulation of gene expression. Successes like nusinersen and ultrapersonalized treatments (for example, Milasen) highlight their potential. Other approaches, including viral gene therapy and CRISPR-Cas9, enable the addition or correction of genes. However, major challenges remain, including high costs, difficulties in conducting clinical trials, and inadequate regulatory frameworks, especially for "N-of-1" therapies. International initiatives are emerging to facilitate access to these innovative treatments in Europe and Switzerland.