Confirmed and hypothetical (?) interplays between HTLV-1 and virus or cell-derived small non-coding RNAs. A. Given the imperfect base-pairing requirements, the multiplicity of cellular miRNAs and the length of the proviral genome, it seems likely (!?) that HTLV-1 is targeted by cellular miRNAs. B. HTLV-1 Tax directly transactivates cellular miRNA genes. Expression of a small non-coding viral RNA by HTLV-1 could affect viral (C) or cellular (D) gene expression through hypothetical (?) mechanisms of RNA interference.