ESR1 and breast cancer: Since mammary tumors in BALB/c-Trp53+/- mice progress from ERα+ lesions to ERα- tumors (K.A. Dunphy, A.C. Blackburn, H. Yan, L.R. O'Connell, D.J. Jerry, unpublished data), the ERα expression observed among mammary tumors in parous mice may reflect an earlier stage of progression due to the longer latencies.