THE XIST LOCUS REPLICATES LATE ON THE ACTIVE-X, AND EARLIER ON THE INACTIVE-X BASED ON FISH DNA-REPLICATION ANALYSIS OF SOMATIC-CELL HYBRIDS

We have recently reported results of DNA replication analysis of three X-linked loci (FRAXA, F8C and XIST) on the X chromosomes in male and female fibroblasts using fluorescence in situ hybridization (FISH) (1) . Although our findings that XIST replicates later on the active X tha n on the inactive X are similar to those of Boggs & Chinault (2) based on a FISH assay in female lymphoblasts, they are the opposite of obse rvations recently reported by Hansen et al. (3) using a different tech nique. Because our conclusions about the inactive X were deduced from the behavior of the active X in male cells, we reexamined the time whe n these loci replicate on the human inactive X chromosome isolated fro m its homolog in somatic cell hybrids. We also studied the same chromo some as an active X in related hybrids. The results provide direct evi dence that the expressed XIST locus on the inactive X replicates earli er than its repressed homolog on the active X and earlier than the FRA XA locus which is repressed on this chromosome. The silent XIST locus on the active X replicates late along with F8C which is also not trans cribed in these cells. Possible reasons for the different results obta ined by Hansen et al. (3) are discussed.