In MMTV-induced mammary tumors, proviral insertions are often found near Wnt gene family members (Wnt1, Wnt3 and Wnt3a), growth factor related genes (Fgf and Fgfr genes, Pdgfr genes and Igf2), R-spondin gene family members (Rspo1, Rspo2 and Rspo3) and mitogen signaling pathway genes (Eras and Map3k8) [11]. Here, RSPO2 is linked to breast cancer.