Trisomy 17 mosaicism
is a chromosomal abnormality in which there are three copies of chromosome 17 in some cells of the body, rather than the usual two copies. Trisomy 17 mosaicism is one of the rarest trisomies in humans. It is often incorrectly called trisomy 17 (also referred to as full trisomy 17), a chromosomal abnormality in which an individual has three copies of chromosome 17 in all of cells of the body. Trisomy 17 has never been reported in any living individuals in the medical literature. Only 23 cases of trisomy 17 mosaicism have been described, most of them have been detected during pregnancy (prenatally) through a test called amniocentesis. In most of these cases, the diagnosis of trisomy 17 mosaicism could not be confirmed in the baby after birth, leading some to propose that the abnormal cells detected in the prenatal test results may have come from tissues that are not part of the fetus in most cases. There have only been a handful of people in whom the diagnosis of trisomy 17 mosacism was confirmed after birth. Because the number of cells with trisomy 17 and the location of the tissues affected differ from case to case, the clinical signs and symptoms (if there are any) vary significantly from person to person. Symptom management is based on the personÕs clinical signs and medical needs. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.