Recombination of VH, DH and JH genes is a unique first step in normal B-cell development. Subsequent differentiation to a mature plasma cell is accompanied by further events in the Ig genes, including VL-JL joining, somatic hypermutation and isotype switching. Chromosomal changes leading to B-cell tumors can occur at many points in this sequence, and may be partly a consequence of the genetic mobility and mutability permitted in order to generate a diverse antibody repertoire. V genes of neoplastic B cells may reflect the point of maturation reached by the B cell of origin, prior to transformation. Analysis of tumors therefore provides useful information on V-gene patterns in normal B cells, and may add another dimension to classification of B-cell tumors. Transformation may also preserve cell populations normally destined to die by apoptosis. Tumor cells arrested in the site where somatic hypermutation and isotype switch are occurring can still be subject to these processes, and could be influenced by persisting antigen. However, mutation is silenced at the point of exit to the periphery, leading to fixed mutational patterns in tumors of mature B cells. V-gene analysis provides an invaluable tool for understanding the genesis of neoplastic change. It also has a clear clinical relevance in tracking tumor cells, measuring residual disease, and finally in offering the opportunity of developing vaccines for treatment.