Monoclonal human B cell tumors are a model system for the study of somatic hypermutation of the Ig genes of humans. It was previously shown that a number of B cell lymphomas exhibited striking V region point mutation, hypothesized to result from the somatic hypermutation mechanism. In this study we have extended the analysis to chronic lymphocytic leukemia. We have cloned and sequenced the productive Vh representing five different cells from a monoclonal chronic lymphocytic leukemia. All five Vh sequences were identical. Therefore, the Vh region in this leukemia was not the subject of detectable somatic mutation. These data suggest that chronic lymphocytic leukemia might lack the mechanism for somatic hypermutation and represent a stage of normal B lymphocyte differentiation in which the somatic hypermutation mechanism is not active.

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