High grade mucinous appendiceal neoplasm (HAMN) is a tumour of the appendix characterized by a mucinous epithelial proliferation. Microscopically the histology is like a low grade appendiceal mucinous neoplasm (LAMN) with the addition of micropapillary and cribriform patterns of growth, piling up of epithelial cells, and marked cytologic atypia. Mitotic figures including atypical forms and necrosis are typically seen. HAMNs commonly harbor KRAS gene mutations, however, GNAS mutations are somewhat less frequent when compared to LAMNs.