Replicase-mediated resistance represents a genetically engineered form of resistance against viral infections and is established by using an intact or modified viral polymerase (replicase) gene for the transformation of the host. The K strain of cucumber mosaic cucumovirus (K-CMV) has been shown to break replicase-mediated resistance generated using a modified RNA2 of the subgroup I strain Fny-CMV. A full-length cDNA clone of K-CMV RNA2 was generated and RNA transcripts were shown to be infectious in combination with Fny-CMV RNAs 1 and 3. We further demonstrate that the ability of K-CMV to overcome resistance in the transgenic plants maps to RNA2. The complete nucleotide sequence of this RNA was determined and sequence differences from Fny-CMV RNA2 are discussed with regard to functional domains and their possible involvement in resistance breakage.