Vaults are ribonucleoprotein complexes highly conserved between species as diverse as Dictyostelium, rat, and human. Thousands of vault particles exist per cell in various tissues including liver, lung, kidney and spleen. These complexes have been found in the cytoplasm as well as associated with the nuclear membrane. Vaults have a large barrel-like middle section with a protruding cap on each end. These structures have an average mass of 12.9 ± 1 MDa, as measured by STEM, and are composed of 3 proteins, p210, pi92 and pi04, as well as an RNA strand of approximately 90 bases. The major vault protein, pl04, constitutes the majority of the particle mass, existing in 96 copies per structure. The major vault protein was recently recognized as the lung-resistance related protein (LRP), a known multidrug resistance gene that is over-expressed in certain cancer cells, including ovarian and myeloid leukemia cells.A three-dimensional structure of the vault complex was generated using the QVIEW and IMAGIC-5 software packages (Fig. 1).