Applied Biological Sciences. In the article “Multiple genetic modifications of the erythromycin polyketide synthase to produce a library of novel “unnatural” natural products” by Robert McDaniel, Arinthip Thamchaipenet, Claes Gustafsson, Hong Fu, Melanie Betlach, Mary Betlach, and Gary Ashley, which appeared in number 5, March 2, 1999, of Proc. Natl. Acad. Sci. USA (96, 1846–1851), the following correction should be noted. A.T. was supported by a fellowship from the National Center for Genetic Engineering and Biotechnology (BIOTEC), the National Science and Technology Development Agency (NSTDA), Thailand.