Pax7::Foxo1 (P7Fm) mice express a Pax7::Foxo1 fusion protein and EYFP after removal of a STOP sequence by Cre recombinase. Specifically, the P7Fm mice have a mouse forkhead box O1 (Foxo1) gene, with an IRES-EYFP sequence inserted into exon 3 of Foxo1, all inserted into a bisected exon 9 of the endogenous paired box 7 (Pax7) gene. In addition, loxP sites flank Pax7 exon 8, the 5' end of exon 9, and a stop signal from the Complement2 (C2) gene. The P7Fm allele will generate a complete transcript of Pax7 before Cre recombination as the STOP cassette blocks expression of FOXO1 and EYFP. When Cre is activated, the floxed region of Pax7 and the stop signal are removed and the second half of exon 9 of Pax7 is set next to exons 2 and 3 of Foxo1, creating an activated Pax7::Foxo1 fusion protein (P7Fa) along with the IRES-EYFP expression. PAX7::FOXO1 is a subtype of alveolar rhabdomyosarcoma which is a soft tissue sarcoma that develops in skeletal muscles. Rhabdomyosarcoma mostly affects children and teenagers but can affect adults as well. Each year, between 400 and 500 people in the U.S. receive a diagnosis of rhabdomyosarcoma. Rhabdomyosarcoma can develop anywhere in the body, but it usually affects muscles in the arms, legs, head and neck, and abdomen (belly). Oncologists can treat and sometimes cure rhabdomyosarcoma, but the condition can come back. P7Fm mice are viable, are normal weight, and are born at Mendelian ratios before Cre recombination. Cancer will not develop in these mice until mated to a Cre-expressing strain, with or without a second mutation such as mutations in the transformation related protein 53 (Trp53) gene. When P7Fm mice are bred to Pax7-Cre mice (Stock No. <a href="https://www.jax.org/strain/010530">010530</a>), resulting Pax7P7Fm/Cre compound heterozygous mice exhibit perinatal hypoplasia of the cranial premaxillary bone and postnatal runting. After Cre-mediated removal of the STOP cassette, EYFP fluorescence should be detectable by FACS.

Pax7::Foxo1 (P7Fm) mice express a Pax7::Foxo1 fusion protein and EYFP after removal of a STOP sequence by Cre recombinase. These mice are useful when studying the PAX7::FOXO1 subtype of alveolar rhabdomyosarcoma.