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The HIV-1 Rev protein facilitates the export of incompletely spliced and unspliced viral mRNAs from the nucleus. Rev polymerizes into two types of filaments in vitro. In the presence of RNA, Rev forms poorly ordered structures, while in the ab-sence of RNA it polymerizes into regular hollow filaments. We have determined the helical structure of the latter filaments by analysis of cryo-electron micrographs, taking into account STEM measure-ments of mass-per-unit-length. They are made up of Rev dimers, arranged in a six-start helix, with 31 dimers in 2 turns, a pitch angle of 45°, and an interstrand spacing of 3.8 nm. Three-dimensional reconstruction at 2.1 nm resolution reveals a smooth outer surface and a featured inner surface, wit