MEDICAL SCIENCES. For the article “Dynamics of HIV-1 recombination in its natural target cells,” by David N. Levy, Grace M. Aldrovandi, Olaf Kutsch, and George M. Shaw, which appeared in issue 12, March 23, 2004, of Proc. Natl. Acad. Sci. USA (101, 4204-4209; first published March 9, 2004; 10.1073/pnas.0306764101), the authors note that Fig. 4 and Table 1, which is published as supporting information on the PNAS web site, presented an overestimation of the percentages of GFP⁺ cells by omitting the GFP⁺ cells from the denominator (the sum of all fluorescent cells) in the calculation of percentages. In addition, the authors have applied a Poisson distribution analysis to account for multiple crossovers within the yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP) genes and to account for crossovers that begin on the YFP gene and end on CFP. By using these methods, the calculated crossover frequencies in the cells analyzed in Fig. 4 have been altered from 5.5, 8.0, and 9.1 to 4.6, 6.7, and 9.6 for HeLa, Jurkat, and PBL, respectively. The conclusions of the manuscript remain unchanged. A more complete description of the new methodology can be obtained by contacting the corresponding author at levy@uab.edu.