Xist yeast artificial chromosome transgenes function as X-inactivation centers only in multicopy arrays and not as single copies

X-chromosome inactivation in female mammals is controlled by the X-inactiva tion center (Sic), This locus is required for inactivation in cis and is th ought to be involved in the counting process which ensures that only a sing le X chromosome remains active per diploid cell, The Xist gene maps to the Xic region and has been shown to be essential for inactivation in cts. Tran sgenesis represents a stringent test for defining the minimal region that c an carry out the functions attributed to the Xic. Although YAC and cosmid X ist-containing transgenes have previously been reported to be capable of ci s inactivation and counting, the transgenes were all present as multicopy a rrays and it was unclear to what extent individual copies are functional. U sing two different yeast artificial chromosomes (YACs), we have found that single-copy transgenes, unlike multicopy arrays, can induce neither inactiv ation in cis nor counting. These results demonstrate that despite their lar ge size and the presence of Xist, the YACs that we have tested lack sequenc es critical for autonomous function with respect to X inactivation.