PF4 and infection: Li et al. [21] demonstrated that repressor C, which they named Pf4r, was responsible for immunity to Pf4 infection and also regulated replication of the Pf4 phage by repressing the expression of xisF4. However, the specific mechanism by which the Pf4r controls superinfection remained unknown in the context of the mutated Pf4r* [14].