Targeted mutagenesis of Tsix leads to nonrandom X inactivation

Cell. 1999 Oct 1;99(1):47-57. doi: 10.1016/s0092-8674(00)80061-6.


During X inactivation, mammalian female cells make the selection of one active and one inactive X chromosome. X chromosome choice occurs randomly and results in Xist upregulation on the inactive X. We have hypothesized that the antisense gene, Tsix, controls Xist expression. Here, we create a targeted deletion of Tsix in female and male mouse cells. Despite a deficiency of Tsix RNA, X chromosome counting remains intact: female cells still inactivate one X, while male cells block X inactivation. However, heterozygous female cells show skewed Xist expression and primary nonrandom inactivation of the mutant X. The ability of the mutant X to block Xist accumulation is compromised. We conclude that Tsix regulates Xist in cis and determines X chromosome choice without affecting silencing. Therefore, counting, choice, and silencing are genetically separable. Contrasting effects in XX and XY cells argue that negative and positive factors are involved in choosing active and inactive Xs.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Chimera
  • DNA, Antisense*
  • Dosage Compensation, Genetic*
  • Female
  • Gene Expression Regulation, Developmental
  • In Situ Hybridization, Fluorescence
  • In Vitro Techniques
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mutagenesis, Site-Directed / physiology
  • RNA, Long Noncoding
  • RNA, Messenger / analysis
  • RNA, Untranslated*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Transcription Factors / genetics*
  • X Chromosome


  • DNA, Antisense
  • RNA, Long Noncoding
  • RNA, Messenger
  • RNA, Untranslated
  • Transcription Factors
  • XIST non-coding RNA