Inactivation of Smad4 in TECs causes thymic hypoplasia. (A) Representative photomicrographs of thymi from neonate (i), 5-week- (ii), and 20-week-old (iii) Smad4^co/co (*) and Smad4^{co/co;Foxn1Cre} (**) mouse littermates. Boxed area is shown in higher magnification to point out cysts in aged Smad4^{co/co;Foxn1Cre} mice. Scale bars represent 1 mm. (B) Smad4 exon 9 deletion was verified in adult MHCII^int and MHCII^hi sorted TEC populations. Rectangles in the dot plots indicate sorting gates. Deletion was quantified by genomic PCR, yielding a 3-kb WT fragment and a 0.5-kb recombined fragment.