Current uses for gene rearrangement analysis in clinical dermatology are listed in Table 3. This technique is useful for determining the existence of clonal populations within a background of polyclonal lymphoid cells; therefore, it is helpful in the diagnosis and staging of patients with CTCL and PTCL. Although dual genotypes do occur, this technique is usually capable of determining lineage in a clonal lymphoid infiltrate and elucidating and characterizing the etiopathogenesis of certain neoplasms. On the basis of this review of the literature and our own experience, we conclude that gene rearrangement analysis shows great promise for monitoring response to therapy and detecting progression or relapse in patients with CTCL and PTCL. With the recent technology of PCR, it is possible to amplify specific sequences of DNA, detect molecular alterations in individual malignant T cells, and even identify exogenous retroviral gene sequences in tissues of patients with CTCL. Although gene rearrangement analysis has supported or established the clonal nature of lymphomatoid papulosis, pre-Sézary syndrome, granulomatous slack skin syndrome, and follicular mucinosis, the clinical significance of these findings is not yet clear. In the case of primary cutaneous B-cell lymphoma and its benign counterpart, B-cell pseudolymphoma, further investigation will be needed to determine the clinical significance of clonal rearrangements.