MST1R and breast cancer: Here, we show that macrophage‐stimulating protein (MSP) and its tyrosine kinase receptor, Recepteur d’origine nantais (RON), are potent drivers of cancer cell growth and tumor progression in a mouse model of TNBC driven by the loss of Trp53 and Brca1. After comparison of two genetically engineered mouse models of TNBC, we found that mammary tumors from K14‐Cre;Brca1F/F;Trp53F/F (KB1P) mice exhibit high endogenous levels of MSP and RON expression.