The sequences of four oat cDNA clones, 3B3-5, 3B3 -7, 3B3T-3 and 3B3T-5, isolated from developing seeds, are all found to possess sequences encoding a characteristic tryptophan-rich domain and, for this reason, have been named tryptophanins. 3B3-3 and 3B3-7 are predicted to encode two similar proteins, each consisting of 147 amino acids. 3B3T-3 and 3B3T-5 are predicted to encode identical proteins, 142 amino acids in length. The oat tryptophanins share sequence identity with two wheat seed proteins, puroindoline and wheat grain softness protein. The tryptophan-rich domains show similarity with the antimicrobial peptide, bovine indolicidin. Copy number reconstruction experiments indicate that the oat tryptophanins are encoded by a multi-gene family. RNA slot blot experiments verify the seed-specific expression of oat tryptophanins while northern blot experiments indicate that cross-hybridizing RNAs are also present in developing wheat, barley, and rye seeds but not in developing rice seeds.