The nonsegmented negative-sense (NNS) RNA genomes of the various MARV isolates range in size from 19,111 to 19,114 nts and contain seven monocistronic genes in a linear order ( Figure 5 ) [66, 67] . Each gene is composed of a highly conserved transcription start and stop signal, an unusually long 3' and 5' untranslated region, and the open reading frame (ORF). The genes are either separated by short intergenic regions that range from 4 to 97 nts, or the transcription stop signal of the upstream gene and the transcription start signal of the downstream gene overlap, sharing five highly conserved nts ( Figure 5 ). The structure of this gene overlap is found among all filoviruses and is unique among members of the order Mononegavirales (for review see [68] ). The 3' and 5' genome ends are extracistronic regulatory regions that contain cis-acting signals essential for transcription and replication, including transcription and replication promoters.