KL-6 is another useful marker in assessing a micropapillary pattern in carcinomas of the breast and urinary bladder, but not the colon.

To evaluate the peculiar "inside-out" pattern in micropapillary (MP) carcinoma, we investigated the usefulness of KL-6 antibody in the assessment of the MP pattern of cancers, in comparison with antibodies to epithelial membrane antigen (EMA), MUC1 (CD227), and CD 10. Immunohistochemical investigation was performed on specimens exhibiting an MP pattern obtained from 12 persons with cancer: 4 with breast carcinoma, 3 with carcinoma of the urinary bladder, and 5 with colonic carcinoma. Immunohistochemical study with KL-6, EMA, and MUC1 antibodies revealed similar continuous linear positive patterns restricted to the surface of the MP pattern in both breast and urinary bladder cancers, revealing the peculiar "inside-out" morphology. However, EMA also gave cytoplasmic positivity in most of the cases tested, and MUC1 was also present in the cytoplasm of some cases. In sharp contrast, immune reactions of colon carcinomas with these antibodies were negative, except for focal positivity for KL-6 and MUC1 antibodies in some cases. CD10 was only focally positive in an MP pattern in 4 of the 5 cases of colon carcinoma and in 1 case with carcinoma of the urinary bladder. These findings suggest that KL-6 is a useful marker to assess the MP character of breast and urinary bladder carcinomas; that MUC1 was similarly positive, with the addition of cytoplasmic positivity in some cases; and that the MP pattern of colon cancer, positive for CD 10, was different in character from both breast and urinary bladder carcinomas, although all these cancers seemingly exhibit similar MP patterns on histopathology. This heterogeneity of the MP pattern in various cancers needs to be investigated when more cases have been accumulated.