In primary sarcomas initiated by Adeno-FlpO from Col1a1FRT-STOP-FRT-Cre-ER-T2; KrasFRT-STOP-FRT-G12D/+; p53FRT/FRT; Rosa26mTmG/+ mice, we were able to activate Cre-ERT2 in tumor cells by intratumoral administration of 4-hydroxytamoxifen or multiple doses of systemic 4-hydroxytamoxifen. This evidence concerns the gene ESR1 and neoplasm.