This family constitutes holin proteins from the dsDNA Siphidoviridae group bacteriophages with two transmembrane segments. Most bacteriophages require an endolysin and a holin for host lysis. During late gene expression, holins accumulate and oligomerize in the host cell membrane. They then suddenly trigger to permeablise the membrane, which causes lysis by allowing endolysin to attach the peptidoglycan. There are thought to be at least 35 different families of holin genes.