Medical Sciences. In the article “Potent inhibition of human immunodeficiency virus type 1 replication by an intracellular anti-Rev single-chain antibody” by Lingxun Duan, Omar Bagasra, Mark A. Laughlin, Joseph W. Oakes, and Roger J. Pomerantz, which appeared in number 11, May 24, 1994, of Proc. Natl. Acad. Sci. USA (91, 5075–5079), the undersigned author wishes to note the following: “I concur with my coauthors that the D8-SFv construct did not have the sequence claimed in our paper. However, I differ from my coauthors in that I believe that the coding DNA could not have derived from the NSI-derived hybridoma cells, because D8-SFv was derived from a sp2/0 fusion partner cell line [Duan, L. & Pomerantz, R. J. (1994) Nucleic Acids Res. 22, 5433–5438], not an NSI fusion partner. Therefore, in my opinion, the D8-SFv aberrant heavy chain could not have come from the sp2/0-derived hybridoma cells, because the sp2/0 myeloma cell line does not express a heavy chain. NSI and sp2/0 cell $lines each express entirely different gene sequences.”