FOXN1 and thymoma: In endogenous mouse models with the specific expression of mutated GTF2I in FOXN1+ thymic epithelial cells, the mutation causes an incomplete block of TEC differentiation with the accumulation of immature TECs and reduced thymopoietic activity [8] and leads to the formation of thymomas resembling WHO type B1 and B2 THs in aged animals [31].