X chromosome inactivation in bone marrow cells of adult mice carrying Searle's X-autosome translocation: occurrence of the early-replicating inactive X chromosome.

Using BrdU labelling-acridine orange fluorescence staining and the expression of PGK-1 isozymes, we attempted to clarify the pattern of X chromosome inactivation in bone marrow cells from adult female mice heterozygous for Searle's X-autosome translocation. The asynchronously replicating X chromosome was always the morphologically normal one, and neither of the translocated X chromosomes showed allocyclic behavior. Unexpectedly, a substantial proportion of the allocyclic X chromosome apparently finished replication earlier than any other chromosomes of the cell. This early replicating allocyclic X chromosomes was judged to be as genetically inactive as the late replicating one, since the Pgk-1b allele on the normal X was never expressed in bone marrow cells. Study of karyotypically normal females and females carrying Cattanach's insertion showed that the early replicating X chromosome is not just a feature of the Searle's translocation in the adult bone marrow cells and may be either paternal or maternal in origin. This type of allocyclic X chromosome deserves attention in assessing X chromosome inactivation in female somatic cells.