The cells of the adaptive immune system, B and T lymphocytes, each generate a unique antigen receptor through V(D)J recombination of their immunoglobulin (Ig) and T cell receptor (TCR) loci, respectively. Such rearrangements join coding elements to form a coding joint and delete the intervening DNA as circular excision products containing the signal joint. These excision circles are stable structures that cannot replicate and have no function in the cell. Since the coding joint in the genome is replicated with each cell division, the ratio between coding joints and signal joints in a population of B cells can be used as a measure for proliferation. This chapter describes a real-time quantitative (RQ-)PCR-based approach to quantify proliferation through calculating the ratio between coding joints and signal joints of the frequently occurring intronRSS-Kde rearrangements in the IGK light chain locus. The approach is useful to study basic B-cell biology as well as abnormal proliferation in human diseases.