The core of late states of maturing human immunodeficiency virus type 1 (HIV-1) has been visualized in three dimensions at approximately 7 nm resolution by electron microscopic tomography. After budding, approximately 25 nm thick precursor material is observed densely assembled inside the viral envelope. Upon proteolysis the core material is transported and condensed in the center of the virion. The core, 100 nm in length, spans the entire diameter of the virion showing a 40-60 nm wide free end and a narrow end approximately 20 nm. A model of the core is derived consisting of two fibers packed into a bilateral, elongated structure. Two ends of the fibers are compacted together, forming one narrow end of the core, while the two other fiber ends are situated more loosely together allowing for flexibility. Structural maturation of the virus could be reflected by the degree of compactness of the core. The narrow end of the core is observed attached to the envelope with a conspicuous core-envelope link (CEL).

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