Table 1.

Lessons from human genetics about viable human trisomies due to silencing by XIST RNA

Condition	Karyotype; Phenotype	Take away
Trisomy X	47, XXX; Little to no phenotype (reviewed in Tartaglia et al 2010)	Presence of two XIST coated Xi or Barr bodies is largely benign Physical presence of an extra chromosome in nuclei is benign
Balanced X: autosomal translocations	46, X, (X:A). Translocation of one female X chr to an autosome with no loss/gain of genetic material, typically has no phenotype. (reviewed in Mattei et al 1982; Schmidt and Du Sart 1992)	Intact X chromosome is typically silenced Lack of silencing of the X:autosomal material is largely attributed to selection against functional monosomy
Unbalanced X: autosomal translocations where the involved autosome is trisomic	Rare case studies in which what could be a lethal trisomy produces a viable mild/moderate phenotype: 47, Y, Klinefelter male with a duplicated X:14 translocation and one X:14 is silenced. (Allderdice et al 1978; Hall et al 2002) 46, female; one intact X and most of the second X chr translocated to most of an extra chr 9. (Leisti et al 1975). XIST RNA was shown to coat much of the chr 9 material (Hall et al 2002) 46, female; one intact X and most of the second X chr translocated to most of the extra chr15. (Stankiewicz et al. 2006)	Rare case reports in which the X: autosome translocation is largely silenced thereby circumventing an otherwise lethal trisomy