RSPO1 and breast cancer: Recently, through high-throughput screening of MMTV insertion sites in mouse mammary tumors, Klijn et al. confirmed that a series of gene loci including Wnt, fgf, fgfr, R-spondin (Rspo), and platelet-derived growth factor receptor (Pdgfr) are directly involved in MMTV-induced mouse mammary tumors (14).