This study describes a young girl with a brain anomaly, eye misalignment, and progressive loss of strength and coordination. Genetic testing revealed a previously undescribed mutation in the KIF21A gene, inherited neither from her mother nor father. Laboratory experiments suggested that this mutation in KIF21A impairs its binding to TUBB3, a protein crucial for nerve cell function and stability, likely leading to her symptoms. By uncovering the effects of this disrupted interaction, this work offers insights into the genetic mechanisms behind certain complex neurological and developmental conditions, highlighting a potential target for future research on treatments and early interventions. (By Nicholas A. Borja, M.D., https://jmg.bmj.com/content/early/2024/12/06/jmg-2024-109908 )