Molecular genetic testing of the FMR1 gene is commonly performed in clinical laboratories. Mutations in the FMR1 gene are associated with fragile X syndrome, fragile X tremor ataxia syndrome, and premature ovarian insufficiency. This document provides updated information regarding FMR1 gene mutations, including prevalence, genotype–phenotype correlation, and mutation nomenclature. Methodological considerations are provided for Southern blot analysis and polymerase chain reaction amplification of the FMR1 gene, including triplet repeat–primed and methylation-specific polymerase chain reaction. In addition to report elements, examples of laboratory reports for various genotypes are also included.