Serotype-1 specific antigen (ssa1) gene could have a pathobiological significance owing to its putative association with phenomenon of stress-precipitated commensal-to-pathogen conversion in the microbial population and suggested genetic correlation between ssa1 and leukotoxin (lkt), in genesis of pneumonic pasteurellosis Thus, PHSSA represent species-specific and virulence-associated gene of M. haemolytica [22]. The gene discussed is TRIM21; the disease is pneumonic pasteurellosis.