Expansion of K111 proviruses in humans took place after the Homo-Pan divergence. (A) Detection of K111 full-length and solo LTR insertions from DNA of New and Old World primates. Full-length K111 proviruses were detected by PCR only in the human and chimpanzee. The 5′ flanking K111 insertions were amplified with the primers P1/P7 and P4, and the 3′ flanking K111 insertions were amplified with the primers ET1 and P6. Solo LTRs were amplified by PCR using the primers P1 and P2 (see Supplemental Methods) and were seen only in humans. Arrows indicate individual insertional polymorphisms. (B) Detection of K111 insertions in human chromosomes. DNA from human–hamster hybrid cell lines, which carry only one specific human chromosome, were analyzed by PCR for the presence of K111 using the set of primers that amplify the 5′ LTR insertion as described. Other bands (e.g., the PCR products detected in chromosomes 3, 6, and 8) were shown by sequencing to be the result of nonspecific PCR amplification.