[FRAXE mental retardation].

The folate sensitive fragile site FRAXE is located in Xq28, 600 kb distal to the fragile X syndrome (FRAXA) fragile site. An unstable GCC triplet repeat responsible for FRAXE exists in the 5' untranslated region of FMR2 gene. Normal alleles range from 10-35 repeats and more frequently 15-20 GCCs. An expanded GCC repeat over 200 is methylated, inactivates FMR2, and results in FRAXE mental retardation. Affected individuals show mild mental retardation and/or autistic symptoms FMR2 is a large gene consisting of 21 exons spanning about 600 kb. FMR2 mRNA is about 9.5 kb and is expressed in the brain(especially in the hippocampus and the amygdala) in adults and also in placenta. The FMR2 protein is 1311 amino acids, contains nuclear localization signals, and is a putative transcription factor.