The HUG gene is a chimeric gene leading to the synthesis of a protein antibody of the gamma (g) isotype consisting of a mouse variable region and a human g1 constant region. In order to produce a functional antibody, gene rearrangement follows a highly regulated process. The heavy chain rearranges first followed by light chain rearrangement. Initially, the signal inhibiting heavy chain rearrangement was thought to be the same as the signal inducing light chain rearrangement. In the HUG transgene, however, the presence of a previously rearranged transgenic g heavy chain inhibits rearrangement of the mouse's endogenous heavy chain in a process called allelic exclusion, but no light chain is formed. We are studying different HUG strains to see if transgenic antibodies will be expressed at different levels, depending on the copy number and locus of the HUG gene in the murine genome. Southern blot analysis was done on DNA isolated from different HUG strains. Blots were probed with a JH probe that had been isolated from the JH11 plasmid that recognized and hybridized to the JH portion of the variable region. This technique allows us to find where the HUG gene had been inserted and how many copy numbers of the transgene are present relative to the endogenous gene, which is present in two copies. The reason for low levels of HUG expression would be explained if the gene had been incorporated in an area where no promoter was present or if the copy number was high, which might hinder transcription.