The test is performed on the patients themselves. The mutation (the defect in the
gene) can be determined in the patient by establishing the gene sequence. If the
defect is found using this method, other members of the family who also carry the
same mutation and who are therefore affected can be identified. The fetus can also
be tested for the defect if one of the parents is affected, allowing for prenatal
If a patient cannot be examined:
It is impractical (but possible) to check family members (or the fetus) directly
using the same methods, which are time-consuming and complex.