Facioscapulohumeral dystrophy (FSHD), one of the most common adult muscular diseases, shows significant clinical heterogeneity that can be partially explained by somatic mosaicism. We here report our findings from the prospective, hospital-based, case-control, observational study of 35 mosaic FSHD patients recruited over 10 years. This largest-to-date mosaic FSHD cohort exhibited significant clinical heterogeneity and relatively slight disease severity. We also demonstrated that both genotypic severity scale and D4Z4 hypomethylation status served as modifiers of clinical phenotypes. Consistent with previous reports, mitotic interchromosomal/intrachromosomal gene conversion without crossover was here identified as a major genetic mechanism underlying these mosaic FSHD patients. (By Dr. Zhiqiang Wang, https://jmg.bmj.com/content/early/2020/03/13/jmedgenet-2019-106638 )