Assembly of human immunodeficiency virus type 1 (HIV-1) particles is initiated in the cytoplasm by the formation of a ribonucleoprotein complex comprising the dimeric RNA genome and a small number of viral Gag polyproteins. Genomes are recognized by the nucleocapsid (NC) domains of Gag, which interact with packaging elements believed to be located primarily within the 5′-leader (5′-L) of the viral RNA. Recent studies revealed that the native 5′-L exists as an equilibrium of two conformers, one in which dimer-promoting residues and NC binding sites are sequestered and packaging is attenuated, and one in which these sites are exposed and packaging is promoted. To identify the elements within the dimeric 5′-L that are important for packaging, we generated HIV-1 5′-L RNAs containing mutations and deletions designed to eliminate substructures without perturbing the overall structure of the leader and examined effects of the mutations on RNA dimerization, NC binding, and packaging. Our findings identify a 159-residue RNA packaging signal that possesses dimerization and NC binding properties similar to those of the intact 5′-L and contains elements required for efficient RNA packaging.