- Human immunodeficiency virus type 1 regulator of virion expression, rev, forms nucleoprotein filaments after binding to a purine-rich "bubble" located within the rev-responsive region of viral mRNAs. Heaphy, S., Finch, J.T., Gait, M.J., Karn, J., Singh, M. Proc. Natl. Acad. Sci. U.S.A. (1991)