Retroviruses use cellular tRNAs as primers for reverse transcription of the viral genomic RNA (Mak and Kleiman 1997). The primer tRNA is selectively packaged during assembly of retrovirus particles. In the case of HIV-1, lysine tRNAs are preferentially incorporated during retroviral packaging, and lysine tRNA 3, the specific isoacceptor form that serves as a primer for reverse transcription, anneals to the PBS (primer binding site) within the U5 region of the viral genomic RNA. This association appears to be mediated by the viral reverse transcriptase (RT) protein, possibly its "thumb" and "connection" domains (Jiang et al. 1993; Mak et al. 1994; Mishima and Steitz 1995).