ESR1 and soft tissue sarcoma: In addition, Cre-ERT2 is tightly regulated by the STOP cassette in Col1a1FRT-STOP-FRT-Cre-ER-T2; Rosa26mTmG/+ mice and, after successful removal of the STOP cassette by Adeno-FlpO, Cre-ERT2 can be efficiently activated by 4-hydroxytamoxifen in primary soft-tissue sarcomas via intratumoral injection or multiple systemic injections to express eGFP in the tumor parenchyma.