The results of previous studies done in our laboratory on breast cancer gene expression profile, using DNA microarrays, led to the discovery of several genes associated with breast cancer progression. Further evaluation of these genes and their involvement at various stages of cancer progression required performance of immunohistochemistry on thousands of different tissue blocks. Tissue microarray (TMA) technology facilitates rapid translation of DNA microarrays results to clinical specimens by using immunohistochemical analysis of protein expression. DNA microarray analysis done in our laboratory showed a significantly higher expression of prostatic-specific antigen (PSA) in invasive ductal carcinomas as compared to ductal carcinoma in situ, a finding contrary to the previously published data for PSA immunoreactivity in breast carcinomas. To find out whether TMA strategy could be used to explore the expression of the candidate genes involved in the breast cancer progression, we constructed a breast cancer progression TMA. It consisted of 2 normal ductal epithelium, 8 ductal carcinoma in situ, 19 invasive ductal carcinomas, and 3 metastatic ductal carcinomas of breast in triplets. Two prostatic adenocarcinomas and 2 normal colons were used as positive and negative controls, respectively. We first used well-documented and well-tested markers, such as antibodies to estrogen receptor, progesterone receptor, and p53. Results of these 3 antibodies were according to the previously published data. To validate our result, we then used antibody to PSA and looked for the expression of this protein on breast cancer progression TMA. Except for the 2 positive controls all 98 cores were found to be negative for PSA expression highlighting the importance of validation studies for DNA microarray results.