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The principal aim of this thesis was to begin to determine the possible functions of a novel and B-cell specific protein, FAM129C, identified from proteomic screening of purified CLL plasma membrane fractions. Bioinformatic analysis showed that FAM129C contained a pleckstrin homology domain that probably causes the protein to be associated with the plasma membrane but lacked any other obvious domains. Using quantitative RT-PCR, I showed that FAM129C was expressed from early stages of B-cell differentiation. It was expressed at high levels in chronic lymphocytic leukaemia (CLL) and in the activated subtype of diffuse large B-cell lymphoma, where it may be a useful diagnostic marker. FAM129C was also expressed at high levels in normal B-cell populations including both naïve pre-germinal centre and memory cell populations but interestingly was rapidly down-regulated following stimulation to proliferation. Similar down-regulation was also observed in CLL cells stimulated to proliferation in vitro. Similar down-regulation was also observed in CLL cells stimulated to proliferate in vitro. Subcellular fraction studies of FAM129C showed wide expression in many different cell fractions, but mainly in the cytoplasm. The pattern of FAM129C expression was similar to that of CXCR4 and, therefore I have speculated that there is a potential association between these two proteins in B cell development and in B cell maturation during germinal centre reaction.