Structural domains comprising this superfamily share the structure of the globular tip protein, known as tail needle, of some tailed bacteriophages such as P22-like phages, which functions as a cell envelope-penetrating device to promote ejection of viral DNA inside the host. In Sf6, a P22-like phage that infects Shigella flexneri, the tail needle folds into a knob with a TNF-like fold, similar to the fibre knobs of bacteriophage PRD1 and Adenovirus PMID:21705802,PMID:23951045.