English: Mammarenavirus structure and genome composition. Mammarenaviruses have a spherical, enveloped structure. The outer surface contains the glycoprotein spike complex GP consisting of the subunits GP1, GP2 and SSP (inset) involved in receptor binding and host cell entry. Within the enveloped structure the small zinc finger matrix protein (Z) encloses the nucleocapsid which is formed of the nucleoprotein (NP) surrounding the small (S) and large (L) RNA segments, and the RNA-dependent RNA polymerase (RdRp) L protein. Mammarenaviruses possess a bi-segmented single-stranded RNA genome. Both RNA segments comprise two open reading frames (ORFs) that are separated by non-coding intergenic regions (IGR) and involved in RNA transcription termination. The S RNA segment encodes the glycoprotein precursor (GPC) and the NP, whereas the L RNA segment encodes the Z protein and the L RNA-dependent RNA polymerase.