As all forms of SMA are caused by mutations in the telomeric survival motor neuron (SMN1) gene (Lefebvre, Burglen, Frezal, Munnich, & Melki, 1998; Lefebvre et al., 1995, 1997), the second copy of SMN (centomeric SMN; SMN2) is an important disease‐modifying gene (Lefebvre et al., 1995). The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.