Carriers of French Canadian descent are more likely to have a mutation called a deletion; this means that a region of the HEXA gene is missing. If you think of the HEXA gene as a page in a book, screening by sequencing is only looking for single letter typos, but can fail to detect missing paragraphs. If you are French Canadian, ask your doctor or genetics professional to order genetic testing that will be able to find a large deletion, if present.