The whole genome sequence of a Boxer dog suggested that the amino acid sequence of the carboxyl terminus of a putative Smad3 is SSVF-(COOH), not SSVS-(COOH) as in all Smad3 sequences identified in many species. Because phosphorylation of the last two serines at the carboxyl terminus is generally indispensable for Smad3-mediated signaling, the role of Smad3 may be unique in dogs. The present study determines the nucleotide sequence of the coding region of canine Smad3 and deduces the carboxyl terminal amino acids of Smad3 in several breeds. Except for the Boxer, the deduced amino acid sequence was SSVS-(COOH) in all dogs examined. In addition, the nucleotide at position 1204 in the Boxer was different from that of the other dogs. Furthermore, there was a SNP at nt 240. The present study indicates that the carboxyl terminal amino acid of canine Smad3 is not unique, although it is unknown in the Boxer breed.