A boss promoter drives expression of sequence encoding the secreted extracellular domain of boss fused to a leucine zipper derived from the Saccharomyces cerevisiae GCN4 gene (GCN4 sequence starts at residue 251, which shortens the α-Helix of the GCN4 coiled-coil domain by two amino acid residues). The presence of the leucine zipper results in the protein produced forming dimers.