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Purpose: Though p53, BRCA1, ATM, PIK3CA, and HER2 genes are shown to be involved in various aspects of breast carcinogenesis, their functional relationship and clinical value are still disputable. We investigated the genetic status or expression profile of these genes to further elucidate their clinical significance. Methods: PCR-SSCP-Sequencing of p53, BRCA1, ATM, and PIK3CA was performed in 145 Bulgarian patients with sporadic breast cancer. Expression profiles of HER2 were determined by ICH and CISH. Relationship between mutations and clinicopathological characteristics was evaluated by Chi-squared and Fisher's exact tests. Multivariate Cox proportional hazard test and Kaplan-Meier analysis were used to evaluate differences in overall survival between groups. Results: The frequency of p53 (22.07%), BRCA1 (0.69%), ATM (7.59%), and PIK3CA (31.25%) alterations and HER2 (21.21%) overexpression was estimated. Mutated p53 was associated with tumor size ( P = 0.033) and grade of malignancy ( P = 0.001), ATM-with grade of malignancy ( P = 0.032), and PIK3CA-with PR-positive tumors ( P = 0.047). HER2 overexpression correlated with age of diagnosis ( P = 0.009), tumor size ( P = 0.0004), and ER expression ( P = 0.011). Univariate survival analysis showed that mutated p53 is an indicator for worse outcome ( P = 0.041). Combination of two genetic abnormalities did not correlate with more aggressive carcinogenesis and worse overall survival. Conclusions: Our data indicated that p53, BRCA1, ATM, PIK3CA, and HER2 alterations specifically correlate with clinicopathological characteristics of Bulgarian patients with breast cancer. Of these genes, only mutated p53 showed significant, though not independent, negative effect on overall survival. [ABSTRACT FROM AUTHOR]