Simple transposons such as (a) above contain palindromic sequences (nucleotide sequences that read the same both forward and backward, such as AAGGAA) at each end, bounded by inverted repeat (IR) sequences on each side. Complex transposons such as (b) above include not only the transposase gene in the insertion sequence, but other genes as well, bounded by insertion sequences. R-factors (genes that carry information about resistance to antibiotics) and cell marker genes are often transposons.
Transposons have been discovered in many organisms, including pathogenic bacteria such as Salmonella typhimurium and protists such as Trypanosoma (pathogens of trypanosomiasis) and Plasmodium (the agents of malaria). Transposons are also responsible for variation in the chemical structure of the five classes of human immunoglobulins involved in humoral immunity to disease.