Breast cancer remains a leading cause of cancer death in women despite widespread screening, in part because screening mammography has high rates of false-negative results and because many women decline to have routine mammograms. The development of sensitive and specific assays for breast tumor markers would improve detection and facilitate screening, diagnosis, therapeutic monitoring and surveillance for recurrence. Nuclear matrix proteins (NMPs) are promising candidates for tumor markers because they are involved in malignant transformation. Therefore, they may be useful for screening and early diagnosis of small tumors. Proteomic analysis was used to demonstrate that a 28.3 kD serum protein, designated NMP66, can distinguish malignant disease from benign conditions and normal controls. NMP66 is now being evaluated as a potential biomarker for early breast cancer detection in large-scale clinical trials.