Furthermore, motoneurons isolated from severe SMA mice (Smn−/−; SMN2), or those depleted of hnRNP-R, show a reduction in clustering of N-type voltage-gated Ca2+ channels (Cav2.2 channels), resulting in reduced Ca2+ transients in neuronal growth cones [124]. This evidence concerns the gene CACNA1B and proximal spinal muscular atrophy.