Retroviral vectors are often used to introduce therapeutic sequences into patients' cells. In
recent years, gene therapy with retroviral vectors has had impressive therapeutic successes,
but has also resulted in three cases of leukaemia caused by insertional mutagenesis, which
has focused attention on the molecular determinants of retroviral-integration target-site
selection. Here, we review retroviral DNA integration, with emphasis on recent genome-wide
studies of targeting and on the status of efforts to modulate target-site selection.