The mutant TECs in our transgenic mice exhibit a gene expression pattern that resembles thymomas of either A-type (elevated levels of Trp53, Xbp1; reduced levels of Myb, Foxm1) or B-type (elevated levels of Myc, Max; reduced levels of Ppara, Rxra) (Fig. 4a); whether this mixed expression pattern is a result of species differences or a consequence of the uniform expression of the mutated Gtf2i gene in all TECs, rather than in a patchy form in the human tumours, is currently unclear. The gene discussed is XBP1; the disease is neoplasm.