A genotype‒phenotype correlation for MSD has been proposed based on experimental data and correlation with the clinical course of MSD patients: SUMF1 mutations severely affecting protein stability and enzymatic function result in more severe forms of MSD, whereas mutations allowing residual enzymatic activity and protein stability were found in attenuated cases (Jaszczuk, Schlotawa, & Dierks, 2017; Sabourdy et al., 2015; Schlotawa et al., 2011). The gene discussed is SUMF1; the disease is mucosulfatidosis.