The thymomas that developed in the Tg1 and Tg4 mice reflect the expression levels of the ΔN64Ctnnb1/ERT2 transgene and share histological appearance (e.g. cellular atypia and perivascular space) and expression patterns of cellular markers (AIRE, p63, p21 and β5t) with those of human thymomas. The gene discussed is MAPK3; the disease is thymoma.