In the great majority of cases, carcinoma of the rectum is an adenocarcinoma, derived from malignant transformation of the columnar rectal epithelium. The more the tumour cells retain normal shape and arrangement (well-differentiated), the less aggressive the behaviour. Conversely, the more cells of an undifferentiated type, the more aggressive the behaviour. Other poor prognostic features include vascular and perineu- ral invasion, the presence of an infiltrating (rather than push- ing) margin and tumour budding. In a small number of cases, the tumour is a primary mucoid carcinoma. The mucus lies within the cells, displacing the nucleus to the periphery, like the seal of a signet ring. Signet-ring carcinomas are rapidly growing, metastasise early and have a poor prognosis.