Breast cancer is the most common cancer in women and despite advancements in diagnostics and treatment methods that have led to lower mortality rates, it remains one of the leading causes of cancer death among women. In general, breast cancer is divided in 5 different subtypes, primarily based on the expression of the hormone receptors the estrogen receptor (ER), progesterone receptor (PR) and the human epidermal growth factor receptor 2 (HER2). Current treatment options for breast cancer patients mainly focus on antagonizing the estrogen and HER2 receptors, leaving patients that do not express any of the these receptors with poor prognosis due to limited treatment options. Furthermore, tumor heterogeneity and resistance towards therapies challenge the efficacy of current therapeutic treatments, thereby negatively influencing the survival rates of breast cancer patients. Therefore, further molecular insight is required to identify novel therapeutic targets to increase treatment options for breast cancer patients. In this thesis, we discussed if R-Spondin (RSPO1-4) proteins could be potential relevant targets for breast cancer. RSPO proteins play important roles during development and in the control of stem cell self-renewal. Consistent with these important roles, aberrant expression of RSPOs has been associated with tumor development in various organs. In this thesis, we examined the oncogenic potential of one of the RSPO proteins, RSPO3, in the breast. We investigated the consequences of increased RSPO3 expression in the breast by using an innovative, new mouse model that increasingly expressed Rspo3, specifically in the mammary glands. We demonstrated that increased expression of RSPO3 consistently results in the formation of poorly differentiated, invasive breast tumors, concluding an oncogenic role for RSPO3 in the mammary gland. We furthermore demonstrated that RSPO3 driven tumors are highly enriched by a luminal progenitor cell population, suggesting that also in the breast RSPO3 could potentially influence stem cell dynamics. Although RSPOs are well known to function as agonists of the Wnt/β-catenin pathway, we found that RSPO3 functions independently of this pathway in breast cancer. Additionally, we aimed to develop a new mouse model for invasive lobular breast cancer by inactivating E-cadherin and P53 in LGR6+ cells that have been identified as tumor initiating cells of luminal, ER+ breast cancer. In this mouse model, we mainly observed the formation of skin tumors characterized as squamous cell carcinomas with expansive or invasive growth. Only one mouse formed an ER negative, invasive ductal breast tumor. Altogether, this thesis contributes to a better understanding on the role of RSPO3 and LGR6 in breast cancer development and progression. With the identification of RSPO3 as an oncogene of breast cancer, opportunities lie ahead to examine RSPO3 as a novel targeted therapeutic option for breast cancer patients.