R-Spondin 1 (RSPO-1) is a secreted growth factor that stabilizes ?-catenin and enhances WNT signaling. R-Spondin 1 is expressed in many tissues and regulates female sex determination, XX gonad development, oocyte differentiation, and hematopoietic stem cell specification. R-Spondin 1 is suggested to function as a stem cell growth factor and may have therapeutic potential in the treatment of gastrointestinal diseases and chemotherapy-induced intestinal mucositis. Additionally, recombinant R-spondin 1 protein is used to initiate and maintain intestinal organoid cultures.