THE MOUSE SB1.8 GENE LOCATED AT THE DISTAL END OF THE X-CHROMOSOME ISSUBJECT 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 mou se Sb1.8 gene is X-linked, as shown by Southern blot analysis and by i n situ hybridization to metaphase chromosomes. Sb1.8 was sublocalized to band F of the mouse X chromosome, distal to Alas2 and proximal to D XPas1, which confirms a region of conservation between band Xp11.21-p1 1.22 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 w as shown to be highly conserved in mammals; partial DNA sequence analy sis indicates 92% identity between the mouse and human genes. In contr ast to the human DXS423E gene, the mouse Sb1.8 gene is subject to X in activation, as shown by restriction enzyme and sequence analysis of mR NA from mice with Searle's translocation (T(X;16)16H). Absence of Sb1. 8 expression from the inactive mouse X chromosome in vitro was confirm ed by analysis of a cell line (Hobmski) in which the M.spretus X chrom osome is inactivated. The Sb1.8 gene is a new member of a group of gen es that escape X inactivation in human, but are inactivated in mouse.