Marker for type sIX effectors domainThis family contains the MIX (Marker for type sIX effectors) domain, a marker of type VI secretion system (T6SS) effectors carrying polymorphic C-terminal toxins. MIX domains have been classified into five clans (called MIX I-V) by Dar et. al. based on sequence similarity. These domains have been further classified as either antibacterial or anti-eukaryotic, based on the presence or absence of adjacent putative immunity genes, respectively. In Vibrionaceae, antibacterial MIX-effectors carrying domains with pore-forming, phospholipase, nuclease, peptidoglycan hydrolase, and protease activities have been identified. Additionally, novel virulence MIX-effectors that employ a combination of antibacterial and anti-eukaryotic MIX-effectors have been found, suggesting that certain bacteria adapted their antibacterial T6SS to mediate interactions with eukaryotic hosts or predators. A subset of polymorphic MIX-effectors, a widespread class of effectors secreted by T6SSs, are horizontally shared between marine bacteria and used to diversify their T6SS effector repertoires, thus enhancing their environmental fitness.