A fragment of cauliflower mosaic virus (CaMV) DNA, containing Δ3, one of the three discontinuity sequences, was cloned in various ways into CaMV DNA deleted for the Δ3 sequence. The series of constructions was monitored for the appearance of the typical single-strand (ss) discontinuity after hybrid CaMV replication in plants. The Δ3 discontinuity was observed only if the orientation of inserted DNA sequence was the same as in the wild-type virus. Long polylinker sequences used for insertion of the fragment into cloned viral DNA, affected the stability of the insert in progeny viral DNA in plants by acting as recombination targets.