Immunoglobulin (Ig) genes undergo V(D)J rearrangement during B cell development. When the heavy chain V(D)J gene segments are successfully rearranged, a heavy chain protein is synthesized. This protein normally then signals the cell to stop heavy chain rearrangement and begin light chain rearrangement. Dr. Roberta R. Pollock's laboratory has developed a transgenic mouse that expresses the HUG ( 1 (human gamma) heavy chain transgene. This transgene inhibits endogenous heavy chain rearrangement. However, while heavy chain expression normally induces light chain V(D)J rearrangement, this does not occur in our transgenic model system. My research focuses on developing an in vitro system to study these signals further. This system consists of hybridoma cells expressing the transgenic heavy chain but not expressing the endogenous heavy chain or the endogenous light chain. We have found by transfecting endogenous heavy chain into the hybridoma cells that this does not induce light chain rearrangement. We believe that this may be due to the fact that these hybridomas are mature B cells which do not normally contain the active RAG-1 and RAG-2 genes. I am transfecting the mature B cell hybridomas with active RAG-1 and RAG-2 genes to determine whether or not that will induce light chain rearrangement.