Diagnosis of OTC deficiency by molecular testing is preferable to the more invasive liver biopsy that is necessary for the enzymatic test. Molecular screening for OTC mutations identifies approximately 80% of mutations, while the remaining undetected mutations are expected to affect promoter function or splicing.7 The OTC gene has an approximately 50:1 male-to-female mutation ratio, and 80% of male probands inherit the mutation from their mothers, while only 23% of manifesting females inherit the mutation. Therefore, any woman who has a son with OTC deficiency has a 20% a priori risk of having another affected son in her next pregnancy. Due to the inheritabil-ity and the severity of the disease with the limited treatment available, molecular screening of at-risk couples for the purpose of prenatal testing may be beneficial. Additionally, mutation identification may be of prognostic value in OTC deficiency (see "Interpretation of Test Results," below).
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