Properties of selected retrovirus transcripts. HIV-1, human T-cell leukemia virus type 1 (HTLV-1), mouse mammary tumor virus (MMTV), and spleen necrosis virus (SNV) transcripts are depicted, including predominant unspliced and spliced mRNA species. Numbering is in reference to the first nucleotide of R, the RNA start site, as +1. Red numbers below each mRNA indicate the nucleotide position of exon junctions. Dashed lines denote introns. AUG indicates translation initiation codon, and black numbers indicate AUG nucleotide positions. The unused AUG in bicistronic transcripts is depicted in gray parentheses. Predicted free energy values are derived from possible RNA structure calculated by Zuker mfold software version 3.2. The number of AUG or CUG codons upstream of the authentic AUG initiator codon is indicated in the far-right column. 7 mG, 5' RNA cap structure; (A)x, poly A tail. HIV-1 information was derived from ; HTLV-1 information was derived from  and GenBank NC_001436; MMTV information was derived from GenBank U40459, DQ223969, and ; SNV information was derived from reference sequence pPB101 .