In earlier models of pore formation, the conserved domain D4 (Figure 3) was thought to be important for toxin targeting, receptor recognition and binding to cholesterol-containing membranes (Farrand et al., 2010; Ramachandran et al., 2002; Soltani et al., 2007a, 2007b), whereas oligomer formation was mainly attributed to intermolecular interactions via D1 and D3 (Lawrence et al., 2015; Ramachandran et al., 2002). It has however been shown that D4 of streptolysin, pyolysin and LLO can oligomerise by itself on cholesterol crystals (Harris et al., 2011; Weis and Palmer, 2001) or erythrocyte ghosts (Köster et al., 2014). Our cryoEM structure confirms that D4 can indeed play a role in oligomer formation through intra- and intermolecular interactions of its loops (Figure 2—figure supplements 2 and 3). Loop β22/23 and the conserved undecapeptide loop with Trp433 at its tip appear to interact with loop β18/19 of the neighbouring monomer, thereby stabilizing the pore complex by inter- and intramolecular interactions (Figure 2—figure supplements 2 and 3).