Fragile site Xq27.3

The gene locus (FRAXA) for the gene (FMR1) is
located on the distal long arm of the X chromosome
in region 2, band 7.3 (Xq27.3). In this region
the great majority of patients and some of
the female heterozygotes show a constriction
(fragile site) in the affected X chromosome in
about 2–25% of metaphases. The constriction
must be induced by folic acid deficiency in the
culture medium, and it must be differentiated
from other fragile sites in this region.