Histopathological analysis of the biopsy specimen confirmed a diagnosis of poorly differentiated invasive adenocarcinoma arising from a background of tubulovillous adenoma with high-grade dysplasia, with intact expression of mismatch repair proteins including MLH1, MSH2, MSH6, and PMS2 (Figure 5). Notably, carcinoembryonic antigen (CEA) levels were markedly elevated at 1666 ng/mL. Here, CEACAM5 is linked to tubulovillous adenoma.