Unlike most short tandem repeats, including the CAG repeat in Huntington’s disease and other polyglutamine diseases, CTG in myotonic dystrophy type 1, CGG in fragile X syndrome-FXTAS, and CCGGGG in C9orf72, in which a significant instability of the expanded repeat was demonstrated,8,48,56-59 the RFC1 AAGGG repeat appears stable across generations, with a repeat size variation, including contraction and further expansion, mostly unchanged or limited to 10% of repeat size. Here, RFC1 is linked to fragile X syndrome.