Breast cancer, the most common malignancy in females, has an estimated 5-10% hereditary predisposition. BRCA1 is a tumor suppressor gene and is known to be responsible for breast cancer and breast-ovarian cancers running in families. In breast caner patients, several mutations in BRCA1 have been reported throughout the gene. This report describes identification of a mutation in BRCA1 gene using protein truncation (PTT) assay in a patient with medullary carcinoma of breast who also had a family history of breast cancer. Following DNA sequencing, the mutation was confirmed as substitution of thymine at position 1123 with guanine of exon 11 (1123 T>G). This mutation can be added to the pool of known BRCA1 mutations in Pakistani population, which will help in developing a local screening panel of BRCA1 mutations.