In the pathogenic bacterium "Neisseria meningitidis" (meningococci), competence for transformation is a highly evolved and complex process where multiple proteins at the bacterial surface, in the membranes and in the cytoplasm interact with the incoming transforming DNA. Restriction-modification systems are abundant in the genus "Neisseria". "N. meningitidis" has multiple type II restriction endonuclease systems. The restriction modification systems in "N. meningitidis" vary in specificity between different clades. This specificity provides an efficient barrier against DNA exchange between clades. Luria, on page 99 of his autobiography, referred to such a restriction behavior as “an extreme instance of unfriendliness.” Restriction-modification appears to be a major driver of sexual isolation and speciation in the meningococci. Caugant and Maiden suggested that restriction-modification systems in meningococci may act to allow genetic exchange among very close relatives while reducing (but not completely preventing) genetic exchange among meningococci belonging to different clonal complexes and related species.