operons), seven ribosomal RNA (rRNA) operons, and 86 transfer RNA (tRNA) genes. Despite having been the subject of intensive genetic analysis for about 40 years, many of these genes were previously unknown. The coding density was found to be very high, with a mean distance between genes of only 118 base pairs. The genome was observed to contain a significant number of transposable genetic elements, repeat elements, cryptic prophages, and bacteriophage remnants. More than three hundred complete genomic sequences of Escherichia and Shigella species are known. The genome sequence of the type strain of E. coli was added to this collection before 2014. Comparison of these sequences shows a remarkable amount of diversity; only about 20% of each genome represents sequences present in every one of the isolates, while around 80% of each genome can vary among isolates. Each individual genome contains between 4,000 and 5,500 genes, but the total number of different genes among all of the sequenced E. coli strains (the pangenome) exceeds 16,000. This very large variety of component genes has been interpreted to mean that two-thirds of the E. coli pangenome originated in other species and arrived through the process of horizontal gene transfer. Gene nomenclature Genes in E. coli are usually named in accordance with the uniform nomenclature proposed by Demerec et al. Gene names are 3-letter acronyms that derive from their function (when known) or mutant phenotype and are italicized. When multiple genes have the same acronym, the different genes are designated by a capital later that follows the acronym and is also italicized. For instance, recA is named after its role in homologous recombination plus the letter A. Functionally related genes are named recB, recC, recD etc. The proteins are named by uppercase acronyms, e.g. RecA, RecB, etc. When the genome of E. coli strain K-12 substr. MG1655 was sequenced, all known or predicted protein-coding genes were numbered (more or less) in their order on the genome and abbreviated by b numbers, such as b2819 (= recD). The "b" names were created after Fred Blattner, who led the genome sequence effort. Another numbering system was introduced with the sequence of another E. coli K-12 substrain, W3110, which was sequenced in Japan and hence uses numbers starting by JW... (Japanese W3110), e.g. JW2787 (= recD). Hence, recD = b2819 = JW2787. Note, however, that most databases have their own numbering system, e.g. the EcoGene