Beyond in vitro studies, tumors from breast cancer cells with high LDLR expression (murine MCNeuA (Her2-positive) and human MDA-MB-231 (triple-negative), respectively) have been incrementally grown in immunocompetent (LDLR–/– and apoE–/–) and immunodeficient (Rag1–/–/LDLR–/– and Rag1–/–/apoE–/–) mouse models of hyperlipidemia with increasing serum LDL concentrations. The gene discussed is RAG1; the disease is hyperlipidemia.