Given that our data shows: (i) M1 promoter expression is detected from plasma cells during in vivo infection; (ii) basal M1 promoter activity requires a functional IRF4 site; and (iii) viral reactivation is linked with plasma cell differentiation, we set out to evaluate whether the M1 promoter is responsive to the MHV68 viral lytic transactivator Rta. Here, IRF4 is linked to infection.