R-Spondin-1 (RSPO1) is the prototype member of the R-Spondin (RSPO) protein subfamily of a superfamily of thrombospondin type 1 repeat (TSR-1)-containing proteins (Chen et al.; Kamata et al.; Kazanskaya et al.; Kim et al.). Although unable to initialize signaling, RSPO family members are potent enhancers of WNT signaling (Cruciat and Niehrs; de Lau et al.; Kamata et al.; Kazanskaya et al.). They are characterized by a TSR-1 domain, a carboxy-terminal region with positively charged amino acids, and two N-terminal furin-like cysteine-rich repeats (Glinka et al.; Kazanskaya et al.). R­-Spondin-1 activates β­-catenin signaling via the WNT signaling cascade and by indirectly increasing low-density lipoprotein receptor-related protein 6 (LRP6) on the cell surface. It does this by binding leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5), and competing with WNT antagonist DKK­1 for binding to the WNT co­receptors, Kremen and LRP­6, which reduces DKK­1-­mediated internalization of LRP6 (Binnerts et al.). RSPO1 is involved in a wide range of pleiotropic roles during embryogenesis, it is required for the specification of hematopoietic stem cells, and it has been shown to be important in the growth, survival, and migration of ovarian cancer cells (Cruciat and Niehrs; de Lau et al.; Genthe and Clements; Liu et al.).