FMR1 and fragile X-associated tremor/ataxia syndrome: However, mosaicism for a PM allele, which is usually associated with ~2–10-fold higher FMR1 mRNA and slightly lower FMRP levels [20], can increase the risks of developing psychotic symptoms [21] and a late-onset neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS, OMIM 300623) [22].