Developmental Biology. In the article “Endothelin-induced conversion of embryonic heart muscle cells into impulse-conducting Purkinje fibers” by Robert G. Gourdie, Yan Wei, Darlene Kim, Sandra C. Klatt, and Takashi Mikawa, which appeared in number 12, June 9, 1998, of Proc. Natl. Acad. Sci. USA (95, 6815–6818), the authors wish to note that the amino acid sequence information for a peptide immunogen used to raise antibodies in rabbits and guinea pigs should be corrected as follows. “These probes were custom produced against a peptide containing conserved residues as 256–270 [originally stated as 2571–271] of rat Cx40 and as 271–280 [originally stated as 272–281] of chicken Cx42 by Research Genetics (Huntsville, AL).”