oxidized, it enhances the effects of UVA, leading to increased membrane lipid oxidation and increased production of reactive oxygen species (ROS). Typically, more altered the levels of 7DHC and cholesterol lead to more severe symptoms of SLOS. The levels of these metabolites also correspond to the severity of the mutation (nonsense versus missense); some mutations of DHCR7 may still show residual cholesterol synthesis, and others may not. However, even individuals with the same mutations or genotype may still show variability in their symptoms. This may be due to maternal factors, such as the transfer of cholesterol to the fetus during pregnancy, as well as the amount of cholesterol present in the brain before the blood–brain barrier forms prenatally. The rate of accumulation and excretion of toxic metabolites may vary from person to person. Maternal apolipoprotein E has also been implicated in individual variability in SLOS, although the exact nature of this relationship is unknown. There are likely more factors contributing to the wide spectrum of effects in SLOS which have not yet been discovered. Screening and diagnosis Prenatally The most characteristic biochemical indicator of SLOS is an increased concentration of 7DHC (reduced cholesterol levels are also typical, but appear in other disorders as well). Thus, prenatally, SLOS is diagnosed upon finding an elevated 7DHC:total sterol ratio in fetal tissues, or increased levels of 7DHC in amniotic fluid. The 7DHC:total sterol ratio can be measured at 11–12 weeks of gestation by chorionic villus sampling, and elevated 7DHC in amniotic fluid can be measured by 13 weeks. Furthermore, if parental mutations are known, DNA testing of amniotic fluid or chorionic villus samples may be performed. Amniocentesis (process of sampling amniotic fluid) and chorionic villus sampling cannot be performed until approximately 3 months into the pregnancy. Given that SLOS is a very severe syndrome, parents may want to choose to terminate their pregnancy if their fetus is affected. Amniocentesis and chorionic villus sampling leave very little time to make this decision (abortions become more difficult as the pregnancy advances), and can also pose severe risks to the mother and baby. Thus, there is a very large desire for noninvasive midgestation diagnostic tests. Examining the concentrations of sterols in maternal urine is one potential way to identify SLOS prenatally. During pregnancy, the fetus is solely responsible for synthesizing the cholesterol needed to produce estriol. A fetus with SLOS cannot produce cholesterol, and may use