The mouse BCM1 (OX45, Blast-1) antigen has been cDNA cloned and sequenced to provide data supporting the view that BCM1, LFA3, and CD2 constitute a subgroup within the Ig superfamily. Mouse BCM1 is widely expressed on leukocytes and is likely to be anchored to the cell surface by a glycosyl-phosphatidylinositol anchor, as is the case for rat and human BCM1 antigen. Genetic linkage studies by recombination and pulse field analysis showed the BCM1 locus (Bcm-1) to be on distal mouse chromosome 1 and to be linked within 1,600 kb to the locus for an ATPase alpha chain gene (Atpa-3). A similar relationship was established between the human BCM1 locus (BCM1) and ATP1A2, and other markers on chromosome 1q. Conservation of genomic organization within a segment of human chromosome 1q and mouse chromosome 1 was demonstrated. A similar situation is seen in the region of the CD2 and LFA3 genes between mouse chromosome 3 and human chromosome 1p. Furthermore, the CD2/LFA3 genes are linked within 580 kb to Atpa-1/ATP1A1 genes to provide a parallel situation to the linkage between Bcm-1/BCM1 and Atpa-3/ATP1A2 on chromosomes 1 (mouse) and 1q (human). Taken together, the data suggest duplication of a chromosome region including the precursors of the genes for BCM1, CD2, and LFA3, and the ATPase genes to give rise to the linkage groups now observed. The duplicated regions may have stayed together on chromosome 1 in the human (with the insertion of a centromere), while in the mouse, the genetic regions are proposed to have become dispersed in the formation of chromosomes 1 and 3. CD2 and LFA3 are more dissimilar in sequence than BCM1 and LFA3, and if the precursors of the CD2 and LFA3 loci formed before the proposed chromosome segment duplication, then a gene encoding a recognizer molecule for BCM1 may exist in linkage with Bcm-1/BCM1 on chromosome 1 (mouse) and 1q (human).

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