NOR包含一系列编码rRNA的基因拷贝,这些基因拷贝位于5对近端着丝粒染色体的短臂上。在中期染色体标本上用银染技术可将NOR显示出来。最近的研究认为一定的NOR变异对Do-wn综合征发病是一个很强的危险因素,特别是dNOR(双倍NOR)与Down综合征发病风险增加20倍及6%的复发风险有关。这种观察对识别可能有Down综合征孩子风险 NOR contains a series of gene copies encoding rRNA, which are located on the short arm of 5 pairs of proximal centromere chromosomes. NOR displays on metaphase chromosomes using silver staining. Recent studies suggest that a certain degree of NOR variation is a strong risk factor for Do-wn syndrome. In particular, dNOR (double NOR) is associated with a 20-fold and a 6% increased risk of developing Down syndrome. This observation is to identify children who may have Down syndrome risk