Premutation alleles within the FMR1 gene are usually unmethylated and have an increased risk of developing a number of clinical phenotypes that fall under the umbrella of FMR1-associated conditions. In this study, we report on a group of premutation carriers who show partial methylation and in some cases we demonstrate the presence of intra and inter-tissue mosaicism. In addition, our findings indicate that the percent of methylation of alleles correlate with the number of clinical symptoms suggesting that methylation may be an indicator for penetrance of the disorder in those cases who present with epigenetic modifications within the gene. (By Dr. Dalyir I. Pretto, http://jmg.bmj.com/content/early/2014/03/03/jmedgenet-2013-102021 )