The DXS423E gene has been localized to Xp11.21 and is expressed in somatic cell hybrids retaining either the human active or inactive X chromosome, demonstrating that DXS423E escapes X chromosome inactivation. The XE169 (DXS1272E or SMCX) gene that escapes X chromosome Inactivation is also located in Xp11.21–11.22 and maps within the same YAC as DXS423E. Thus the DXS423E and XE169 genes define a new region in the proximal short arm of the X chromosome that is not subject to X chromosome inactivation, supporting a regional basis for escape from inactivation.