Biochemistry. In the article “BAP-135, a target for Bruton’s tyrosine kinase in response to B cell receptor engagement” by Weiyi Yang and Stephen Desiderio, which appeared in number 2, January 21, 1997, of Proc. Natl. Acad. Sci. USA (94, 604–609), due to a printer’s error, the communicated line was omitted from the manuscript. This line should read: Communicated by Max D. Cooper, University of Alabama at Birmingham, Birmingham, AL, November 5, 1996 (received for review September 23, 1996).