Streptolysin O, a 63 kDa exotoxin coded by slo gene, is a well-characterised thiol-activated cytolysin, which damages cholesterol-containing membranes resulting in disruption and lysis of the target cell. On the basis of homology model and secondary structure analysis, the toxin has four domains of which domain 4 is of particular importance and is directly linked to domain 2 by a glycine linker and remained consistent in initial membrane recognition. Domain 4 reduces the hydrophobic ratio when compared with its template, which would affect the activity of the toxin at low pH.