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标记染色体或超数结构异常染色体(ESACs),是小的超数染色体,通常缺少识别带型,不能凭借常规的G带分析。ESACs可能与畸形或发育异常有关,但在临床上正常人也可见到。在产前研究中,大约有0.8～1.5/1000胎儿可找到,而发生率似乎随孕母年龄而增加。大约40％是家族性的,若父母携带者为正常表现型,后代异常的风险低。但重新发生的ESACs往往给遗传咨询造成困难。最大的难题是作出ESACs的诊断而不能确定其来源,往往不能告知双亲确切的
Marker chromosomes or super structural abnormalities (ESACs), which are small, super-chromosomes, usually lack recognition bands and can not be characterized by conventional G-banding. ESACs may be associated with deformities or dysplasia, but are clinically normal. Approximately 0.8 to 1.5 / 1000 fetuses are found in prenatal studies, and the incidence seems to increase with gestational age. About 40% are familial, and if the parents are carriers of normal phenotype, the risk of anomalous offspring is low. However, recurrent ESACs often cause difficulties in genetic counseling. The biggest challenge is to make a diagnosis of ESACs without identifying their source, often without informing the parents exactly