is based on the name of the corresponding gene or extein. For example, the intein found in Thermoplasma acidophilum and associated with Vacuolar ATPase subunit A (VMA) is called "Tac VMA". Normally, as in this example, just three letters suffice to specify the organism, but there are variations. For example, additional letters may be added to indicate a strain. If more than one intein is encoded in the corresponding gene, the inteins are given a numerical suffix starting from 5′ to 3′ or in order of their identification (for example, "Msm dnaB-1"). The segment of the gene that encodes the intein is usually given the same name as the intein, but to avoid confusion the name of the intein proper is usually capitalized (e.g., Pfu RIR1-1), whereas the name of the corresponding gene segment is italicized (e.g., Pfu rir1-1). Types of inteins The type of the splicing proteins is categorized into four classes: maxi-intein, mini-intein, trans-splicing intein, and alanine intein. Maxi-inteins are N- and C-terminal splicing domains containing an endonuclease domain. The mini-inteins are typical N- and C-terminal splicing domains; however, the endonuclease domain is not present. In trans-splicing inteins, the intein is split into two (or perhaps more) domains, which are then divided into N-termini and C-termini. Alanine inteins have the splicing junction of an alanine instead of a cysteine or a serine, in both of which the protein splicing occurs. Full and mini inteins Inteins can contain a homing endonuclease gene (HEG) domain in addition to the splicing domains. This domain is responsible for the spread of the intein by cleaving DNA at an intein-free allele on the homologous chromosome, triggering the DNA double-stranded break repair (DSBR) system, which then repairs the break, thus copying the intein-coding DNA into a previously intein-free site. The HEG domain is not necessary for intein splicing, and so it can be lost, forming a minimal, or mini, intein. Several studies have demonstrated the modular nature of inteins by adding or removing HEG domains and determining the activity of the new construct. Split inteins Sometimes, the intein of the precursor protein comes from two genes. In this case, the intein is said to be a split intein. For example, in cyanobacteria, DnaE, the catalytic subunit α of DNA polymerase III, is encoded by two separate genes, dnaE-n and dnaE-c. The dnaE-n product consists of an N-extein sequence followed by a 123-AA intein sequence,