In a multigenerational pedigree, cooccurrence of a pathogenic <i>SMCHD1</i> variant exacerbated hypomethylation and clinical severity.<h4>Conclusions</h4>Hypomethylation of the distal D4Z4 array serves as a robust biomarker for phenotypic penetrance and disease progression in borderline-allele FSHD. The gene discussed is SMCHD1; the disease is facioscapulohumeral muscular dystrophy.