The CSEIL/CAS protein (CAS) is a Ran-binding protein with a function as a nuclear transport (export) factor. CSEIL/CAS, similar to Ran and other ran-binding proteins, plays at the same time an important role in the mitotic spindle checkpoint, which assures genomic stability during cell division. This checkpoint is frequently disturbed in neoplasms of various origin, including breast, hepatic and colonic tumors. CAS is located on chromosome 20ql3 and amplified in several cell lines, including breast, colon and bladder cancer. MEKl phosphorylation is known to be a reason for different CAS localization and activity. We evaluated the expression of CAS in 50 benign and malignant tumors of the breast by immunohistochemistry. Benign lesions of the breast (n=13) revealed a weak, predominantly cytoplasmatic CAS positivity. In ductal and lobular in situ carcinomas (n=17), 70-90% of the tumor cells were positive for anti-CAS staining which was predominantly cytoplasmatic. In invasive ductal and lobular carcinomas (n =20), 70-90% of the tumor cells stained positive with anti-CAS in a predominantly nuclear pattern. Different localization of CAS might affect its role not only for chromosome segregation, proliferation and apoptosis, but also its function in nuclear transport of proteins like retinoblastoma-gene-product, p53 and BRCAl. A different regulation in this checkpoint might contribute to the invasive potential in malignant carcinomas of the breast. Alteration of CAS-activity, possibly via MEKl-inhibition, might therefore be a possible option for breast cancer therapy.