In the final preparation of the manuscript of our report "Regulation by ATP and ADP of CFTR chloride channels that contain mutant nucleotide-binding domains" (18 Sept., p. 1701) (1), we inadvertently plotted the data for figure 1C with an incorrect x axis: MgATP was plotted on the x axis instead of P(o). We did not immediately notice the error, which was brought to our attention by Charles Venglarik and Robert Bridges, because the shape of the two curves is similiar. The correct plot is shown in the figure below. In both plots the data do not fit a straight [See figure in the PDF file] line, which supports our interpretation that more than one site may be involved with adenosine triphosphate (ATP) regulation of the cystic fibrosis transmembrane conductance regulator (CFTR). We regret any inconvenience this may have caused.