Desmin (DES) is a key protein that helps muscle cells maintain their structure and function. In this study, we identified and characterized a previously unknown mutation in DES in an Argentine family affected by a muscle disease. Unlike most known DES mutations, this alteration is a small insertion whose disease-causing role was unclear. By combining clinical data, advanced computer modeling, and laboratory experiments, we demonstrated that this variant disrupts how desmin molecules assemble, leading to abnormal protein aggregation in muscle cells. This work provides evidence for a disease mechanism in desmin-related disorders and improves the interpretation of rare genetic variants. (By Dr. Lucia N. Moro, https://jmg.bmj.com/content/early/2025/12/31/jmg-2025-110852 )