The mouse Sb1.8 gene located at the distal end of the X chromosome is subject to X inactivation.

The mouse homolog of the human DXS423E (SB1.8) gene has been isolated by screening a mouse cDNA library. Like its human counterpart, the mouse Sb1.8 gene is X-linked, as shown by Southern blot analysis and by in situ hybridization to metaphase chromosomes. Sb1.8 was sublocalized to band F of the mouse X chromosome, distal to Alas2 and proximal to DXPas1, which confirms a region of conservation between band Xp11.21-p11.22 in human and band XF in mouse. In situ hybridization also showed that the Smcx (Xe169) gene maps near Sb1.8 in band F. The Sb1.8 gene was shown to be highly conserved in mammals; partial DNA sequence analysis indicates 92% identity between the mouse and human genes. In contrast to the human DXS423E gene, the mouse Sb1.8 gene is subject to X inactivation, as shown by restriction enzyme and sequence analysis of mRNA from mice with Searle's translocation (T(X;16)16H). Absence of Sb1.8 expression from the inactive mouse X chromosome in vitro was confirmed by analysis of a cell line (Hobmski) in which the M. spretus X chromosome is inactivated. The Sb1.8 gene is a new member of a group of genes that escape X inactivation in human, but are inactivated in mouse.