Biochemistry. In the article “Clonal selection and in vivo quantitation of protein interactions with protein-fragment complementation assays” by Ingrid Remy and Stephen W. Michnick, which appeared in number 10, May 11, 1999, of Proc. Natl. Acad. Sci. USA (96, 5394–5399), the following corrections should be noted. There were several errors in the references listed in the text. On page 5394, refs. 4–6 should be read 5–7, ref. 7 should be read 4, and ref. 6 should be read 7. On page 5397, ref. 22 should be read 23.