To date, only few mutations in the RAD51 gene were associated to Congenital Mirror Movements (CMM), a rare genetic disorder for which affected patients cannot perform unimanual or asymmetric bimanual movements. In our study, identification of new mutations in RAD51 allowed to confirm haploinsufficiency of RAD51 in CMM and to demonstrate that compensation at the protein levels can explain the incomplete penetrance of CMM. Moreover, the polymerization of mutated RAD51 proteins was altered. These molecular and cellular results represent a new step in deciphering why mutations in this gene are associated with CMM or other pathologies in humans. (By Drs Caroline Dubacq and Oriane Trouillard, https://jmg.bmj.com/content/early/2023/06/11/jmg-2023-109189 )