Treatment of breast carcinoma MCF7 cells with estradiol led to induction of a new 27kD protein, the cDNA was later cloned and identified as p27. This highly hydrophobic protein of 122aa has 33% overall sequence similarity to the product of the 6-16 gene, that is induced by alfa/beta type interferons. It has been shown that p27 is transcriptionally induced by interferon-alpha in various human cell lines. The induction induced by interferon alpha is independent of the presence of estradiol receptors on the cells. High levels of p27 mRNA, detected by Northern blotting, was found in 50% of the primary breast carcinoma cells, suggesting p27 can be a diagnostic marker for breast carcinoma detection. Examination of p27 expression by in situ hybridization in p27 over-expressing tumors suggest that p27 gene is localized in cancer cells and sometimes also in fibroblastic cells of tumor stroma. IFI27 mRNA expression is highly up-regulated in lesional psoriatic epidermis and in non-lesional keratinocytes. It was also expressed in lichen planus, chronic eczema, cutaneous squamous cell cancers, and during normal wound repair IFI27 was found in the proliferating subpopulation of keratinocytes. Further studies are now necessary to elucidate the cause of p27 gene overexpression in breast carcinoma and in particular to determine whether it corresponds to chromosomal rearrangements in the 14q32 region and/or to induction by interferons of the alpha/beta type.