To test whether the triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me) and the rexinoid LG100268 (268) prevent the formation of estrogen receptor (ER) – negative mammary tumors or either arrest the growth or cause regression of established tumors in MMTV-neu mice. The gene discussed is ESR1; the disease is breast cancer.