Recently, differentiation related to the thymic/ultimobranchial sac has been postulated in the histogenesis of CMTC, given the co-expression of CK5 and CD5 in the morular component, together with positivity for CDX2 (recently detected in a subset of thymic carcinomas) [111, 113, 115–117]. The gene discussed is CD5; the disease is thymic carcinoma.