Primary sclerosing mucoepidermoid carcinomas with eosinophilia (SMECE) of the thyroid gland are rare tumors that can present diagnostic difficulties to the pathologist due to the unusual histologic features. Furthermore, the etiology of these tumors has been debated in the literature, with some authors believing that the tumors arise from remnants of the ultimobranchial body (UBB, solid cell nests) and others proposing that they arise from follicular epithelial cells. Because SMECE often occur in glands with chronic lymphocytic thyroiditis and UBB hyperplasia, and do not stain like follicular or parafollicular cells, it is likely that the tumors do arise from UBB/solid cell nests. In this study, we provide additional evidence for this relationship, by demonstrating that SMECE stain strongly positive for p63, which is a new marker for UBB/solid cell nests.