Purpose: We analyzed the expression of critical cell cycle regulators cyclin E and cyclin D1 in familial breast cancer, focusing on BRCA mutation^negative tumors. Cyclin E expression in tumors of BRCA1 or BRCA2 carriers is higher, and cyclin D1 expression lower, than in sporadic tumors. In familial non-BRCA1/2 tumors, cyclin E and cyclin D1expression has not been studied. Experimental Design: Cyclin E and cyclin D1 immunohistochemical expression was studied in tissue microarrays consisting of 53 BRCA1, 58 BRCA2, 798 familial non-BRCA1/2, and 439 sporadic breast tumors. Results: In univariate analysis, BRCA1 tumors had significantly more frequently high cyclin E (88%) and low cyclin D1 (84%) expression than sporadic (54% and 49%, respectively) or familial non-BRCA1/2 (38% and 45%, respectively) tumors. BRCA2 tumors had significantly more frequently low cyclin D1 expression (68%) than sporadic or familial non-BRCA1/2 tumors and significantly more frequently high cyclin E expression than familial non-BRCA1/2 tumors. In a logistic regression model, cyclin expression, early age of onset, and estrogen receptor (ER) and human epidermal growth factor receptor-2 (HER2) status were the independent factors most clearly distinguishing tumors of BRCA1mutation carriers from other familial breast cancers. High cyclin E and low cyclin D1 expression were also independent predictors of BRCA2 mutation when compared with familial non-BRCA1/2 tumors. Most interestingly, lower frequency of high cyclin E expression independently distinguished familial non-BRCA1/2 tumors also from sporadic ones. Conclusions: Cyclin E and cyclin D1 expression distinguishes non-BRCA1/2 tumors from both sporadic and BRCA1-and BRCA2-associated tumors and may reflect different predisposition and pathogenesis in these groups.High-penetrance predisposition genes cause 5% to 10% of breast cancer. The two major high-penetrance susceptibility genes BRCA1 and BRCA2 account together for f20% of familial breast cancers in Finland (1). Cancers in carriers of BRCA1 and BRCA2 mutations differ from sporadic and familial non-BRCA1/2 cancers. BRCA1-associated tumors are of higher tumor grade; more often estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) negative, and p53 positive; and more often of medullary histology than sporadic cancers (2 -6).cDNA expression analysis suggests that the basal cell epithelial phenotype is overrepresented in tumors of BRCA1 mutation carriers (7). This phenotype has high grade, is ER/PR/ HER2 negative, often of typical or atypical medullar histology, and expresses cytokeratin 5/6, cyclin E, and epidermal growth factor receptor (8 -10). Expression of basal keratins cytokeratin 14 and cytokeratin 5/6, together with ER status, seems to be predictive for BRCA1 mutation status compared with sporadic breast tumors (11).In most studies to date, BRCA2-associated tumors have shown a phenotype between the BRCA1-associated and sporadic tumors (4,5,12). cDNA studies, however, identify a di...