R-Spondin 3 (RSPO3, roof plate-specific spondin 3), also called cysteine-rich and single thrombospondin domain containing-1 (Cristin 1), is an ~31kD secreted protein that shares ~40% aa identity with the other three R‑Spondin family members (1, 2). All are positive modulators of Wnt/ beta -catenin signaling, but each has a distinct expression pattern (1-4). Like other R‑spondins, R‑Spondin 3 contains two adjacent cysteine-rich furin‑like domains (amino acids (aa) 35-135) with one potential N‑glycosylation site (aa 36), followed by a thrombospondin (TSP-1) motif (aa 147-207) and a region rich in basic residues (aa 211-269). Only the furin-like domains are needed for beta -catenin stabilization (2). Within aa 21-209, mouse R‑Spondin 3 shares 93%, 97%, 96%, 95% and 91% aa identity with human, rat, equine, bovine and canine R‑Spondin 3, respectively. Potential isoforms of 217, 224 and 252 aa are divergent or truncated at the C terminus; the 252 aa form also lacks aa 4‑33 at the N‑terminus (5). Mouse R‑Spondin 3 is critical for development of the placental labyrinthine layer, probably by promoting VEGF expression and thus vascular development (6, 7). It is also essential for expression of the placenta-specific transcription factor, Gcm1. In the mouse embryo, R‑Spondin 3 is often expressed by or located near endothelial cells (6). It is found in the roof plate, tail, somites, otic vesicles, cephalic mesoderm, truncus arteriosus, atrioventricular canal of the developing heart, and strongly but transiently in developing limbs (4, 7). R‑Spondins regulate Wnt/ beta -catenin by competing with the Wnt antagonist DKK-1 for binding to the Wnt co-receptors LRP-6 and Kremen, reducing their DKK‑1‑mediated internalization (8, 9). Reports differ on whether R‑Spondins bind LRP-6 directly (8-10). R‑Spondin 3 has also been identified as an oncogene (11).