Recent studies indicate that, unlike DM1, either CLCN1 or SCN4A acts as genetic modifiers in DM2 patients since mutations/polymorphisms in these genes may contribute to exaggerated phenotype with more severe muscle stiffness and myotonia (Suominen et al., 2008; Cardani et al., 2012; Bugiardini et al., 2015; Peddareddygari et al., 2016; Binda et al., 2018). This evidence concerns the gene SCN4A and myotonic dystrophy type 1.