Cloning of a complementary DNA encoding a new mouse B lymphocyte differentiation antigen, homologous to the human B1 (CD20) antigen, and localization of the gene to chromosome 19. 
The human B1 (CD20) molecule is a differentiation Ag found only on the surface of B lymphocytes.  This structurally unique phosphoprotein plays a role in the regulation of human B cell proliferation and differentiation.  In order to determine whether this structure is also expressed by murine B cells, cDNA clones that encode the mouse equivalent of the B1 molecule were isolated.  The longest murine cDNA clone isolated, pmB1-1, contained a 1.4-kb insert with an 873 base pair open reading frame that encodes a protein of 32 kDa.  The predicted mouse B1 protein contains three hydrophobic domains that may span the membrane four times and shares a 73% amino acid sequence homology with the human B1 protein.  The pmB1-1 cDNA probe was used to examine mB1 mRNA expression.  Northern blot analysis indicated that pmB1-1 hybridized with two mRNA species of 2.3 and 3.0 kb that were expressed only in murine spleen lymphocytes, in B lineage cell lines representing mature B cells, and were weakly expressed in one of two plasmacytoma cell lines.  pmB1-1 failed to hybridize with RNA isolated from murine T cell lines, thymus, and nonlymphoid tissues.  Southern blot analysis indicated that mB1 was encoded by a single copy gene.  In situ hybridization localized the mB1 gene to chromosome 19 band B, a region that also contains the genes that encode the Ly-1, Ly-10, and Ly-12 Ag.  These results suggest that only B cells express this heretofore undescribed murine cell-surface protein that is structurally homologous with the membrane-embedded human B1 Ag.