Integrating vector systems used in clinical gene therapy have proven their therapeutic potential in the long-term correction of immunodeficiencies^,,,. The integration loci of such vectors in the cellular genome represent a molecular marker unique for each transduced cell and its clonal progeny. To gain insight into the physiology of gene-modified hematopoietic repopulation and vector-related influences on clonal contributions, we have previously introduced a technology—linear amplification–mediated (LAM) PCR—for detecting and sequencing unknown DNA flanking sequences down to the single cell level (Supplementary Note online). LAM-PCR analyses have enabled qualitative and quantitative measurements of the clonal kinetics of hematopoietic regeneration in gene transfer studies, and uncovered the clonal derivation of non-leukemogenic and leukemogenic insertional side effects in preclinical and clinical gene therapy studies^,,,. The reliability and robustness of this method results from the initial preamplification of the vector-genome junctions preceding nontarget DNA removal via magnetic selection. Subsequent steps are carried out on a semisolid streptavidin phase, including synthesis of double complementary strands, restriction digest, ligation of a linker cassette onto the genomic end of the fragment and exponential PCR(s) with vector- and linker cassette–specific primers. LAM-PCR can be adjusted to all unknown DNA sequences adjacent to a known DNA sequence. Here we describe the use of LAM-PCR analyses to identify 5′ long terminal repeat (LTR) retroviral vector adjacent genomic sequences (Fig. 1 and Box 1).