Cauliflower mosaic virus (CaMV) is the type member of the caulimoviruses, one of the six genera in the Caulimoviridae family, pararetroviruses that infect plants (Pringle, 1999). Pararetroviruses replicate through reverse transcription just like retroviruses, but the viral particles contain DNA instead of RNA (Rothnie et al., 1994).

The CaMV particle is an icosahedron with a diameter of 52 nm built from 420 capsid protein (CP) subunits arranged with a triangulation T = 7, which surrounds a solvent-filled central cavity (Cheng et al., 1992). It contains a circular double-stranded DNA molecule of about 8.0 kB, interrupted by sitespecific discontinuities resulting from its replication by reverse transcription. After entering the host, the single stranded nicks in the viral DNA are repaired, forming a supercoiled molecule that binds to histones. This DNA is transcribed into a full length, terminally redundant 35S RNA and a subgenomic 19S RNA.

The promoter of the 35S RNA is a very strong constitutive promoter responsible for the transcription of the whole CaMV genome. It is well known for its use in plant transformation. It causes high levels of gene expression in dicot plants. However, it is less effective in monocots, especially in cereals. The differences in behavior are probably due to differences in quality and/or quantity of regulatory factors. The promoter was named CaMV 35S promoter ("35S promoter") because the coefficient of sedimentation of the viral transcript, whose expression is naturally driven by this promoter, is 35S. It is one of the most widely used, general-purpose constitutive promoters. It was discovered at the beginning of the 1980s, by Chua and collaborators at the Rockefeller University.