Growth arrest and DNA damage-induced 45, alpha (GADD45A) is a candidate breast cancer susceptibility gene because its product participates in DNA repair and it is a downstream gene of p53 and BRCA1, both of which are breast cancer susceptibility genes. We screened germline mutations of GADD45A in 185 non-BRCA1/2 familial breast cancer patients, but no deleterious mutation was found. Seven single-nucleotide-polymorphisms were identified in a subsample. Five common variants (minor allele frequency > 10%) were genotyped for association analyses to scrutinize the relationship between breast cancer and polymorphisms in GADD45A in two independent population sets (total n = 1,861). In the first case-control study (n = 1,457, cases 820, controls 637), a comparison of genotype frequencies between sporadic breast cancer patients and controls indicated the CT/TT-genotypes of +1506C>T and CG/CC-genotypes of +3204G>C were associated with decreased breast cancer risk (adjusted odds ratio (OR), 0.77; 95% confidence interval (CI), 0.62-0.96; and adjusted OR, 0.71; 95%CI, 0.57-0.88, respectively) compared with their wild-type homozygotes. A common haplotype CGTCC was also associated with reduced risk (P = 1.0 x 10(-4)). In a second familial breast cancer patient-based case-control study (n = 404, cases 185, controls 219), although +1506C>T and +3204G>C failed to be validated, the haplotype CGTCC showed a borderline significance. Notably, the combined P-values were robust for +3204G>C (P = 3.1 x 10(-4)) and CGTCC (P = 1.6 x 10(-5)). Moreover, CGTCC was correlated with a higher GADD45A expression in normal breast tissues. In conclusion, although germline mutations of GADD45A is not common in familial breast cancer patients, polymorphisms/haplotypes in GADD45A contribute to breast cancer risk, at least to sporadic breast cancer.