Although the vast majority of SMA cases result from a large deletion in SMN1 (Crawford and Pardo, 1996; Kariya et al, 2008; Lefebvre et al, 1995), 1% of SMA cases are caused by loss-of-function missense mutations, some of which are located in the Tudor domain (Cusco et al, 2004; Kotani et al, 2007; Selenko et al, 2001; Takarada et al, 2017). Here, SMN1 is linked to proximal spinal muscular atrophy.