A series of herpes simplex virus type 1 derivatives, having a sequences composed of DR1, Ub, (DR2)3-7, DR4t (a truncated form of DR4), and Uc were isolated and examined. The derivative having a sequences with six copies of DR2 generated progeny viruses having a sequences with the same number (six copies) of DR2. Another derivative, having a sequences with three and seven copies of DR2, generated progeny viruses having a sequences with varied numbers (4, 5, 8, and 10 copies) of DR2, besides the original DR2 arrays (three and seven copies). Therefore, the variation in copy number of DR2 was assumed to be caused mainly by recombination between DR2 arrays rather than by slippage within a DR2 array during DNA replication. The presence of DR2-like sequences in internal direct repeat elements of DR4 and DR3.5 supported the hypothesis of the recombinogenic property of DR2. The equal distribution of divergence of a sequences to both ends of the virus genome favors the double-strand break and gap repair model to explain gene conversion and amplification of the a sequence
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