The vir (virulence) region lies outside the T-DNA on the Ti plasmid. The 30-40 kb vir region is a regulon of 8 operons. Out of these 8 operons, 6 are essential and 2 are non-essential. The essential operons are virA, virB, virC, virD, virE and virG; the 2 non-essential operons are virF and virH. Each operon is made up of different number of genes. virA, virG and virF have 1 gene; virE, virC and virH have 2 genes; virD has 4 genes; virB is the largest operon and has 11 genes. The virA and virG operons are constitutively expressed. They code for a two-component (VirA-VirG) system and activate the transcription of other vir genes.