A phylogenetic tree of circulating Belgian virus isolates. Molecular phylogenetic analysis of full genome nucleotide sequences was constructed using the Neighbor-Joining method. Phylogenetic relationships were estimated using the Clustal Omega method. Prototype subtype 1 (LV) and subtype 3 (Lena) PRRSV-1 strains, vaccine parental (DV) and attenuated (MLV-DV) PRRSV-1 strains were added to phylogenetic tree. The optimal tree is drawn to scale. Numbers indicate bootstrap values of 100 replicates. Strain nomenclature is as follows: name or diagnostic number/year of isolation/country of origin/GenBank accesion number.