A 25-year-old, gravida 1, para 0, woman underwent amniocentesis for cytogenetic and single-nucleotide polymorphism (SNP) array analysis at 18 weeks of gestation because of the increased Down syndrome risk of 1/13, calculated from a low maternal serum alpha fetoprotein (AFP) level of 0.820 multiple of median (MoM), a low uE3 level of 0.178 MoM, and a high human chorionic gonadotropin (hCG) level of 4.574 MoM in the second trimester. The gene discussed is AFP; the disease is Down syndrome.