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The XLR sequence family encodes RNA transcripts specific to late-stage T and B cells and their neoplasms. Only one apparently functional mRNA has been identified thus far and this encodes a novel 25 kDa nuclear protein. In this report, we find that the XLR gene family is composed of 50-75 copies per haploid genome which localize to at least two different portions of the mouse X chromosome. Neither of these locations are near the xid mutation that earlier work had correlated with XLR. In addition, some members of this family are also on the Y chromosome. Another surprising finding is that while the fourteen genomic clones examined to date have the same exon-intron structure and are closely related with respect to sequence conservation (90%), all appear (in most cases by multiple criteria) to be non-functional, raising the possibility that all but one of the members of this large semi-dispersed family are pseudogenes.