The fragile site

The fragile site associated with FRAX–A may be detected
using cytogenetic methods by culturing cells in the absence of
folic acid and thymidine but this is not a sensitive test for
detecting carrier females. The expansion of the CGG repeat in
the FMR-1 gene may be detected at the DNA level using PCR.
After amplification, the size of the repeat from each
chromosomal copy is determined by polyacrylamide gel
electrophoresis. Samples with a known number of repeats are
used as size standards. This type of approach can be used only
as a screen to detect normal sized alleles. Because full
mutations with long stretches of CGG repeats are too large to
amplify effectively, Southern blotting is still widely used in