The Tat and Rev proteins of HIV-1 and the Rex protein of HTLV-I do not interact with their cognate ligands via a particular structural motif but instead specifically recognize RNA molecules by using agglomerations of arginine residues (1). These proteins are members of the so-called arginine-rich motif (ARM) family. There is little data to support (or contradict) the hypothesis that a few simple arginine:RNA interactions govern how ARMs recognize their viral targets. Not only is it unclear how ARM proteins other than Tat interact with their cognate RNA ligands, for the most part it is not even known how structurally complex these RNA ligands are. In order to fully explore the range of RNA sequences and structures that can bind to ARMs we have carried out in vitro genetic selections with two disparate viral proteins: Rev and Rex.