Psoriasin (S100A7) is a member of the S100 gene family that is involved in psoriasis, which can be induced in cultured squamous epithelial cells. Previously, this molecule has been shown to be over-expressed in human ductal carcinoma in situ and invasive breast cancers. This current study examine the levels of expression of this molecule in a group of human breast tumours, with particular emphasis on the relationship with the clinical outcomes using quantitative PCR and the distribution in tumours using immunohistochemistry. Psoriasin is primarily expressed in breast cancer cells, and at very low level in normal breast epithelial cells. Quantitative analysis of psoriasin mRNA has shown that breast tumour tissues exhibited a significantly high level of psoriasin compared with normal tissues (p=0.0026). The levels do not correlate with nodal status of breast tumours, however levels in grade 2 and grade 3 tumours were significantly higher compared with that in grade 1 tumours (p=0.07 and p=0.0015, vs grade 1 respectively). Lobular carcinoma also had higher levels of psoriasin compared with ductal tumours. The most interesting observation is that levels of psoriasin were significantly higher in patients who developed metastatic disease and in patients who died of breast cancer (p=0.02 and p<0.001, vs disease-free, respectively). It is concluded that aberrant expression of psoriasin is commonly seen in human breast cancer and that excessively high levels are correlated with the clinical outcomes.

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