FMR1 and fragile X-associated tremor/ataxia syndrome: These STRs are generally categorized based on their repeat lengths into three groups: normal, intermediate (characterized by a repeat size below the pathogenic length but exhibiting genetic instability, with the potential for expansion in further generations), premutation [with 55–200 GGC repeats of FMR1 gene, carrying an increased risk for fragile X tremor/ataxia syndrome (FXTAS)] and pathogenic expansions.