Follicular lymphoma (FL) is generally an incurable disease. In each case the tumor is composed of a clonal population of B cells with a unique B cell antigen receptor (BCR). We have recently discovered that at the time of diagnosis, each FL tumor contains a subpopulation of tumor cells that has defective signaling through its B cell antigen receptor (1). The size of this BCR insensitive tumor subpopulation expands over time, and it predicts poor response to chemotherapy and a shorter overall survival. Thus, we pose the question "what is the relationship between the two subpopulations of tumor cells in follicular lymphoma- the BCR sensitive and the BCR insensitive?" We postulate several models:1. The BCR insensitive population arises from the BCR sensitive population, 2. Each of the two tumor subpopulations arises from a common but rare tumor-initiating cell, or 3. The BCR insensitive population is the more primitive cell that initially gives rise to the dominant BCR sensitive population but eventually dominates due to negative selective forces of antigenic stimulation or differential sensitivity to therapy. We will distinguish between these hypotheses by separating the two subpopulations of tumor cells from individual cases and by performing comparative genetic analyses on the respective pairs. We will clone and sequence the VDJ region genes from each subpopulation and examine their patterns of V region somatic mutations. By comparing their respective evolutionary paths we will infer the clonal relationships between the BCR sensitive and BCR insensitive subpopulations. In a second approach we will compare the global patterns of DNA gains and losses between the two tumor subpopulations and identify the changes unique to each. All tumor cells should share the driver DNA gains and losses with the progenitor tumor cell; additional gains and losses should reveal how the two subpopulations evolved in relation to each other. Finally, we will search for the root cause of BCR insensitivity in each case. Using next generation DNA sequencing technology, we will conduct large scale deep resequencing of genes of the BCR signaling pathway. We expect to find mutations in the proximal members of the pathway. In order to discover other differences between BCR sensitive and BCR insensitive tumor cell subsets we will compare their gene expression profiles.