English: The genome organization and RNA synthesis of Mononegavirales. The negative-sense NNS genome is depicted from the 3′ end to the 5′ end, showing the 3′ leader (Le; cyan box), genes (gray, blue, or green box) flanking with gene start (GS; white box) and gene end (GE; black box), and 5′ trailer (Tr; yellow box). The essential genes (N, P, and L) and necessary cofactors (M2 or p30) for RNA synthesis are colored in blue and green, respectively. The RNA-dependent RNA polymerase (RdRP) sequentially produces a gradient level of Le RNA (red line) and viral mRNAs (black, blue, or green line), with the attenuation of the downstream mRNAs at each gene junction. The Le RNA (red lines inside the box) remains uncapped and nonpolyadenylated, while the viral mRNAs are 5′ capped, methylated, and 3′ polyadenylated. The lines under the Le RNA and representative viral mRNAs indicate the abundancy and gradient levels of the RNA transcripts. The promoters for transcription and replication are shown with magenta arrows.