Reversible cell cycle arrest and block of virus replication indicate non-covalent docking of JMN3-003 to its target structures, which is corroborated by the compound's stability, low chemical reactivity profile and the complete absence of virucidal activity in pre-incubation settings. An inhibition profile of JMN3-003 closely mimicking that of AS-136A, the pathogen-directed blocker of MeV RdRp targeting the viral L polymerase protein [36] , and the block in viral RdRp activity in the context of viral infection and minireplicon reporter assays by JMN3-003 consistently point towards interaction of the compound with a host cofactor essential for RdRp function as the basis for its antiviral activity. While viral RdRp depends on a variety of host cell components [1] , unperturbed cellular mRNA synthesis and, thus, uninterrupted host RNA polymerase function in the presence of compound exclude interference of JMN3-003 with essential transcription initiation factors.