Microbiology. In the article “Activation of the JC virus Tat-response transcriptional control element by association of the Tat protein of human immunodeficiency virus 1 with cellular protein Purα” by Chavdar P. Krachmarov, Lara G. Chepenik, Sharon Barr-Vagell, Kamel Khalili, and Edward M. Johnson, which appeared in number 24, November 26, 1996, of Proc. Natl. Acad. Sci. USA (93, 14112–14117), the authors wish to note that the sequences of two deletion mutants were incorrectly described. In the left column on page 14116, lines 13–16 should read: “We have tested the effect of two additional Purα mutants on transactivation of the JVC late promoter in vivo. One mutant, Purα 1-215, is illustrated in Fig. 3. The other mutant was PurαΔ72-216.” The last three sentences of this paragraph should be deleted. Please note that the experiments mentioned in this correction were not included in any of the figures or tables and were not mentioned elsewhere in the text. The essential results and conclusions as stated in the Abstract are unchanged.