Here, we show that short-term treatment with galunisertib (LY2157299), a small molecule inhibitor of TGFBR1 (TGFBRi)15, reduces the frequency of mammary tumors induced by estrogen in inbred ACI rats16 and by NMU (N-Nitroso-N-methylurea) exposure in outbred SD rats17. The gene discussed is TGFBR1; the disease is breast cancer.