My research program has focused on the DNA sequences that mediate antibody gene rearrangement events, especially heavy (H) chain class switch recombination. Through examination of the MPCll mouse myeloma cell line and its variants, we have observed another recombination event, not confined to antibody genes, namely, unequal sister chroinatid exchange (USCE). We propose to generate a universal substrate for assaying recombinogenic sequences, including those that mediate H chain class switching and those involved in USCE. The MPCll USCE event involves TC and TG dinucleotide tracts, each of which is capable of adopting an unusual DNA conformation for TC, a triplestranded structure, and for TG, left-handed DNA. We propose to examine the DNA conformation of the juxtaposed TC and TG dinucleotide tracts at the site of the USCE event. We have begun analysis of MPCll variant cells that are apparently and interestingly unstable in that they show rearrangements of both constant and Variable regions, implying some independence from feedback regulation. Initial study of one variant has led to the observation that sequences lying 3' of the Ca gene undergo rearrangements in several myeloma and hybridoma cell lines. We propose experiments to investigate the functional and structural significance of these 3'beta rearrangement events to the organization and expression of the Ig cluster. Our long term goal is to understand how patterned Ig rearrangements may be governed and how promiscuous rearrangements underlying various malignancies may be avoided.