TABLE 3.

Summary of Xist knockout experiments

Study	Deletiona	Studied in:	Findings
Penny et al. (128)	7 kb of exon 1 and 36-bp promoterb	ES cells and chimeric embryos	Differentiated ES cells: X bearing the mutant Xist remains active and X bearing the normal Xist is inactivated in 60% of cells; both Xs are active in the remaining cells
			Chimeric embryos: X bearing the mutant Xist remains active, and X bearing the normal Xist is inactivated in the ES cell-derived cells
Marahrens et al. (98)	Exons 1–5c	Mice	Germ cells: male with the mutant Xist is able to produce viable, fertile sperm
			Somatic tissues: X bearing the mutant Xist is not inactivated, but dosage compensated by inactivation of X bearing the normal Xist; cells with both Xs active may be eliminated during development.
			Extraembryonic tissues: suggested that lethal when Xp bears the mutant Xist, possibly due to inability to inactivate Xp; viable when Xm bears the mutant Xist

^aIn both studies, the Xist gene is replaced with the neomycin resistance gene by homologous recombination. 

^bAn Xist deletion was created on 129-derived X chromosome in a female XX ES cell line (PGK12.1). 

^cThe deletion was created on 129-derived X chromosome in a male XY ES cell line (J1).