The 1-LTR circle can be formed by either of two pathways. The first involves a failure to complete reverse transcription; the second, annotated here, follows the completion of reverse transcription and is mediated by cellular enzymes. In this pathway, the action of host cell homologous recombination enzymes on the long terminal repeat (LTR) termini of the viral DNA results in formation of a single LTR. This reaction probably takes place after partial or complete disassembly of the PIC to expose the viral DNA. Repair of this intermediate as in the late stages of homologous recombination pathways results in formation of the 1-LTR circle. Mutations in the Mre11/Rad50/NBS pathway influence the formation of 1-LTR circles.