The transgene encodes a membrane-bound, but not a secreted, immunoglobulin mu heavy chain. The variable region of the transgene comprises the VDJ moiety from the functionally rearranged Igh gene of the C57BL/6-derived B1-8 hybridoma, containing the V-region segment Vh186.2 and recognizing the hapten (4-hydroxy-3-nitrophenyl)acetyl. This is followed by a J-C intron fragment containing the Igh intronic enhancer and an Igh-6 constant region gene (of haplotype a) from which the exon encoding the secreted form of Igmu and its associated polyadenylation signal have been deleted.