(A) Schematic of inducible alleles in mice, which depend on Cre-mediated recombination of the rtTA and the TetO promoter–controlled SS18-SSX2 allele (hSS2T), plus the presence of doxycycline for expression of the fusion oncogene. (B) Fluorescence photomicrographs showing some GFP from the recombination of rtTA and more GFP from the activation of hSS2T in embryonic fibroblasts in culture. Scale bars: 100uM. (C) Schematic and Kaplan-Meier plots of the absence of tumorigenesis in 5 groups with noted genotypes and doxycycline or postnatal induction with tamoxifen as well as rates of tumorigenesis in 2 groups with embryonic induction of SS18-SSX2 expression (n = 25 per group; log-rank test Z score = 4.48 aænd P < 0.001, comparing 2 mg and 1 mg dosing). (D) Gross image, (E) GFP fluorescence image, and (F) H&E histology photomicrographs of a limb tumor that developed in a 6-month-old Myf5Cre
hSS2T mouse after embryonic initiation of doxycycline (1 mg/ml). Open arrowheads indicate biphasic histologic features of mucinous gland formation. Scale bar: 100 μm.