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The p73 gene is a structural and, in overexpression systems, functional p53 homologue. Ectopic p73 expression can activate a broad subset of p53-responsive genes, induce apoptosis, and act as a growth suppressor. Yet, viral oncoproteins that antagonize p53 (adenovirus E1B 55K, SV40 large T, and human papillomavirus E6) do not antagonize p73. This could suggest that inactivation of p73, in contrast to p53, is not required for tumorigenesis. Also, p73 is not activated by DNA damage. Because intragenic p73 mutations in tumors have not been reported and imprinting is idiosyncratic, tumor-specific changes in wild-type p73 expression levels become the most reliable guide toward identifying the normal function of p73 and its role in tumorigenesis. We analyzed 77 invasive breast cancers and 7 breast cancer cell lines for p73 mRNA expression levels, allelic origin, intragenic mutations, and COOH-terminal splice variants. A range of normal tissues, including breast, showed very low p73 expression, with little variation from tissue to tissue. In contrast, 38% (29 cases) of breast cancers had elevated p73 mRNA ranging from 5-25-fold above normal, with the remaining tumors (64%) falling within the normal range. Moreover, five of seven cell lines (71%) also exhibited p73 overexpression (13-73-fold). Yet, no correlation with p21 mRNA and protein levels was present, although four of the five lines were mutant for p53. Mutation analysis of the eight highest expressers showed wild type status. Eight of 14 informative samples were biallelic, whereas the remaining 6 samples showed monoallelic expression. Tumors and cell lines with p73 overexpression tended to exhibit a complex profile of up to six different COOH-terminal splice variants, whereas normal and transformed tissues with low p73 mRNA predominantly expressed p73 alpha. We confirm the previously described variants p73 gamma and delta in breast tissue and describe two novel isoforms, p73 epsilon and phi, thereby further enlarging combinatorial possibilities. Together, our in vivo data show that p73 does not have a role as a classic Knudson-type tumor suppressor in breast cancer.