The X-linked lymphocyte-regulated (Xlr) protein is a 30,000 Mr nuclear protein 
bearing homology with meiosis-specific proteins and expressed in late stage B 
lymphoid cell lines. In the present study we investigated its expression in the 
T lymphoid lineage. In adults, a high level of expression was detected in 
CD4-CD8- thymocytes. Most remarkably, the peak of Xlr expression occurred early 
during thymus cell ontogeny, precisely on days 14-15 of gestation, and was 
associated with the first wave of pre-T cell differentiation. Its onset preceded 
the rearrangement of TCR genes, as Xlr expression was conserved in thymus cells 
from RAG1(0/0) mice. The lower expression of Xlr on day 13 of fetal development, 
the bright Thy1+ phenotype of Xlr-positive cells, their large size, and their 
absence from subcapsular areas suggest that Xlr expression must be turned on 
within the thymus and not in prethymic precursors. From day 16 of gestation, Xlr 
expression decreased markedly. At birth and later, Xlr(high) cells were mostly 
large cells scattered throughout the cortical area. As shown by confocal 
microscopy, expression of Xlr closely overlapped that of SATB1, which binds 
special AT-rich DNA sequences associated with the nuclear matrix and plays an 
important regulatory role for many genes. The remarkably regulated expression of 
Xlr in the lymphoid cell lineage and of its homologue Xmr in the germ cell 
lineage suggests that they might play an important role in chromatin metabolism 
at critical stages of differentiation during which the genome undergoes 
irreversible rearrangements.