All the NeoR⁺/HSV-tk⁻ clones from the same patient descend from the same circulating GMC. All 31 NeoR⁺/HSV-tk⁻ clones were screened with the 4 retroviral insertion-specific PCRs. PCRs were performed with a LTR-specific reverse primer and a forward primer complementary to the genomic sequence where the retroviral sequence was inserted, previously identified after cloning/sequencing of the LAM PCR product. A PCR product of the expected length was detected in every clone of 1 patient and was absent in the 3 others. Vertical lines have been inserted to indicate a repositioned gel lane.