Exon organization and conserved domain structure of three medaka R-spondins. The medaka Rspo1 and 2 protein genes share a common organization, each consisting of five coding exons corresponding to predicted structural domains. Predicted domains include a leading signal peptide (Sig. Pep), two furin-like type Cys-rich domains (FU1, FU2), a thrombospondin-type domain (TSP) and low complexity (white color). Additionally, Rspo3 protein possesses an extra furin-like type Cys-rich domain (FU3) located in exon 4. Conserved domain residues and exon boundaries are indicated by rectangle (gray or black) and dotted lines, respectively.