Toeprinting assays fail to provide information on more detailed structural changes that the mRNA might undergo upon binding to the ribosome. Therefore, a footprint assay was performed to obtain structural information upon the interaction of mSP-SL-HIV-1 mRNA with 70 S ribosome-tRNA complexes. In these complexes the ribosome is positioned with the slippery sequence at the decoding site. Results of the chemical probing with KE, 1-cyclohexyl-2-morpholino-carbo-diimide-bmetho-ptoluene sulfonate (CMCT) and DMS are shown in Figure 4 . For free mSP-SL-HIV-1 mRNA the chemical modification patterns and levels of reactivities are identical to those previously published (9) . Nucleotides in the apical ACAA teraloop as well as A +43 and A +44 from the bulge are found to be reactive to DMS (Figure 4) , while nucleotides G +41 and G +42 were reactive to KE. In the lower stem most of the guanine, adenine and uracil bases are accessible to the chemical probes indicating poor stability (9) .