Occludin is an integral membrane protein localised at tight junctions (TJs). It has become clear that the TJ is an important structure that cancer cells must overcome in order to metastasize successfully. The purpose of this study was to elucidate the importance of the expression of occludin in human breast cancer. Human tissues and breast cancer cell lines were amplified for functional regions of occludin. Tumour tissues showed truncated and/or variant signals. There was also considerable variation in the expression of occludin in the 10 human breast cancer cell lines investigated. Western blotting demonstrated that variants in the MDA-MB-231 and MCF-7 human breast cancer cell lines did not fit the expected occludin signals for changes in phosphorylation status. Immunostaining showed similarly disparate levels of expression. Ribozyme knockdown resulted in increased invasion, reduced adhesion and significantly reduced TJ functions. Q-RT-PCR analysis of 124 tumour and 33 background human breast tissues showed occludin to be significantly decreased in patients with metastatic disease. Immunohistochemical staining showed a decreased expression of occludin in the tumour sections. This study demonstrates for the first time that occludin is differentially expressed in human breast tumour tissues and cell lines. This loss of or aberrant expression has clear repercussions as to the importance of occludin in maintaining TJ integrity in breast tissues. Such inappropriate expression could play a part in breast cancer development.