PilE polymers form the structural core of pili, which are immunogenic and highly variable hair-like surface organelles required to initiate infection through attachment to epithelial cells. New pilE variants arise frequently through RecA-dependent non-reciprocal gene conversion events using silent pilS genes as donors or occasionally through pilE deletion events and through transformation. Flanking regions are thought to be important in providing homologous target sequences for gene conversion, and gene conversion is almost exclusively one-way and is thought to follow a mini-cassette exchange model involving crossing over within conserved pil regions. pilE genes share a conserved 150bp segment at the 5' end and contain six variable segments at the 3' end, flanked by short conserved segments. Silent PilS genes are found in 5 or so clusters throughout the chromosome, each containing 1-6 pilS genes. They share the variable and intervening conserved 3' segments with pilE genes, but lack a promoter, ribosomal-binding site, and the 5' region found in pilE genes.