Fetal sex identification in maternal plasma using short tandem repeats on chromosome X

Analysis of fetalDNA in maternal plasma has recently been introduced as a newmethod for noninvasive prenatal diagnosis. In the majority of cases, the Y chromosome–specific sequences are commonly used as a fetus-specific marker with a high risk of falsenegative cases. We attempted to develop a sensitive and reliable X chromosome short tandemrepeat (STR)multiplex PCR amplification systemthat is suitable for the amplification of short-sized templates of free fetal DNA. Because of specific characteristics of fetal DNA in maternal plasma, cell-free fetal DNA is smaller than corresponding maternalDNA, and sowe selected 10X-STRloci inwhich the allele sizewas 250 bp. In addition, fetal sex was also investigated using the amelogenin gene in the same multiplex assay. Twenty-six women were enrolled in the study. Eight of 26 total fetuses analyzed were male and 18 were female. In the whole sample, X-STRs were informative with a mean of 4.84 ± 1.43. A mean of 2.67 ± 1.28 X-STRmarkers per sample (range 1–5) of paternally inherited fetal alleles were detected in pregnant women carrying a female fetus. In all cases, blind determination of fetal sex by means of the identification of amelogenin and X-STR markers was confirmed by fetal karyotyping. This study showed that this noninvasive technique is a reliable and accurate tool to investigate free fetal DNA in pregnancies within the first trimester and could be widely used in clinical research and diagnosis.