This family represents a conserved region found in many bacterial porlymorphic toxins which is located before the C-terminal toxin modules [1][2].

1: A novel immunity system for bacterial nucleic acid degrading toxins and its recruitment in various eukaryotic and DNA viral systems. Zhang D, Iyer LM, Aravind L; Nucleic Acids Res. 2011;39:4532-4552. PMID:21306995

2: A widespread family of polymorphic contact-dependent toxin delivery systems in bacteria. Aoki SK, Diner EJ, de Roodenbeke CT, Burgess BR, Poole SJ, Braaten BA, Jones AM, Webb JS, Hayes CS, Cotter PA, Low DA; Nature. 2010;468:439-442. PMID:21085179