Schematic of HERV-K(HML-2) provirus and splicing of
mRNAs. Irrelevant proviral regions are omitted. rec mRNA is generated by a second splicing event of env mRNA removing most of the env gene region. Both rec and np9 mRNA utilize the same splice acceptor site just upstream of the 3′LTR. Because SD2 for rec mRNA is located within the 292-bp sequence missing in HML-2 type 1 proviruses, an alternative np9-specific SD2 is used instead also resulting in a translation frameshift when spliced to exon 3. Location of PCR primers used for amplification of rec and np9 mRNA/cDNA is indicated. Note that the lower provirus map is not drawn to scale. LTR, long terminal repeat.