By definition, minigenes are short open-reading frames (ORF), usually encoding approximately 9 to 20 amino acids, which are expressed in vivo (as distinct from being synthesized as peptide or protein ex vivo and subsequently injected). The in vivo synthesis confers a distinct advantage: the expressed sequences can enter both antigen presentation pathways, MHC I (inducing CD8+ T- cells, which are usually cytotoxic T-lymphocytes (CTL)) and MHC II (inducing CD4+ T-cells, usually 'T-helpers' (Th)); and can encounter B-cells, inducing antibody responses. Three main vector approaches have been used to deliver minigenes: viral vectors, bacterial vectors and plasmid DNA.