CAV-2 has four early replication area, among them E3 district is the non- essential district to replicate, it will not influence the replication of virus to lack or insert the foreign gene of other species.

• CAV-2有四个早期转录区,其中E3区是病毒复制的非必需区,缺失或插入外源基因不会影响病毒的复制。