MUC5AC and signet ring cell carcinoma: Saito and Iwaki [5] reviewed all the mucin-producing adenocarcinomas of the prostate reported in the English literature and described three subtypes: (1) mucinous carcinoma; (2) signet ring cell carcinoma, defined as having signet ring morphologic structure in at least 25% of the tumor volume; (3) mucinous carcinoma with signet ring cells, defined as having extracellular mucin in at least 25% of the tumor volume or having signet ring morphologic structure in less than 25% of the tumor volume.