Lysis protein S; also called ''pinholin'' or pinholin S(21)68, of 71 aas and 2 TMSs. It forms small heptameric pores that depolarize the membrane (Park et al., 2007; Pang et al., 2009).  This holin is of topological Class II, forming 2 TMSs, with the N- and C-termini inside (Park et al. 2006). TMS1 partially externalizes from the lipid bilayer regulates channel-formation and interacts with the membrane surface, whereas TMS2 remains buried in the lipid bilayer in the active conformation and forms the pore (Ahammad et al. 2019). Pinholin S(21)68 triggers the lytic cycle of bacteriophage phi21 in infected Escherichia coli. Activated transmembrane dimers oligomerize into small holes and uncouple the proton gradient. Structural models have been proposed for (1) the oligomeric pinhole (right-handed heptameric TMD2 bundle), (2) the active dimer (right-handed Gly-zipped TMD2/TMD2 dimer), and (3) the full-length pinholin protein before being triggered (Gly-zipped TMD2/TMD1-TMD1/TMD2 dimer in a line) (Steger et al. 2020). The TMSs are α-helical rather than pi or 310-helices which have been observed in other channel forming proteins (Drew et al. 2021). Pinholin S(21) mutations induce structural, topological and conformational changes (Ahammad et al. 2021).