APPLIED BIOLOGICAL SCIENCES. For 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 authors note the following. In Materials and Methods, under the subheading “Construction of Replacement Cassettes,” the forward oligonucleotide for PCR amplification of rapAT2 should read 5′-TTTAGATCTGTGTTCGTCTTCCCGGGT, where the italicized residues indicate a BglII site. In Fig. 2, compound 43 should be omitted, and the structure of compound 40 is incorrect as drawn; it should contain a single bond between C10-C11 and a ketone at C5.