Mucinous infiltrating invasive ductal adenocarcinoma consists of 2-4% invasive breast cancer, but is a very interesting type due to its macroscopic similarity to non-special-type (NST) ductal carcinoma. The macroscopic similarity of mucinous and infiltrating ductal carcinoma NST adenocarcinomas consists of a loose and edematous stroma, which is often seen in portions of NST carcinoma and may mimic the mucin pools of mucinous carcinoma. In this study the authors examined the ultrastructural differences between mucinous carcinoma and infiltrating ductal carcinoma NST. They also examined the protein expression of the tissues by 2D electrophoresis due to their belief that from the results of these two levels it is possible to understand the changes that take place both in the ultrastructural and biochemical levels in these two types of breast cancer. The ultrastructural results from mucinous carcinoma have shown many changes in cytoplasmic organelles in comparison to normal samples, depending on the grade and the number of metastatic lymph nodes. At the 2D elecrophoresis level the authors studied two interesting polypeptides, calreticulin and thioredoxin. Both of these proteins were found in patterns of fibroadenoma, mucinous carcinoma, and NST carcinoma, but with different quantitative expression among them. In the future the quantitative differences of these two proteins may provide specific tumor markers for these two types of carcinoma.