The primary structure of two nodule specific soybean genes are presented. The two genes code for primary products of 20.0 (nodulin 20) and 22.7 (nodulin 22) kdaltons, respectively. Both genes are related to the nodulin 23 and 44 genes. Alignment of the deduced amino acid sequences of all four genes revealed three domains of high homology interrupted by highly diverged regions due to numerous duplication and insertion events. The first conserved domain codes for a putative signal peptide, while the two others each contain four Cys residues that can be arranged in a way reminiscent of the metal binding domains present in some enzymes and in several DNA binding proteins.