SMN1 and proximal spinal muscular atrophy: To explore whether Munc13-1 mRNAs undergo Smn-dependent axonal translocation in spinal motoneurons, we utilized the severe Taiwanese-SMA mouse model on a C57BL/6 J background (C57BL/6 J.29P2-Smn1Hung<tm1Msd > /J)60,61.