Toxic component of a type I toxin-antitoxin (TA) system (Probable). When overexpressed kills cells within minutes; causes collapse of the transmembrane potential and arrest of respiration (PubMed:10361310, PubMed:20105222). Its toxic effect is probably neutralized by antisense antitoxin RNA SokA (PubMed:10361310). May be repressed by MqsA. Interrupted by an IS150 element, which silences the locus in strain K12 MG1655. Belongs to the Hok/Gef family.