Acquired or inherited mutations of tumor suppressor genes are crucial in the initiation and progression of human malignancies. The tumor suppressor gene BRCA1 (Breast Cancer Susceptibility gene 1) is responsible for a significant portion of inherited breast and ovarian cancers. It encodes a protein 1863 aminoacids. It has a zinc finger domain, two putative nuclear localization signals, a leucine zipper and a transactivation domain in the C-terminus region. This region acts as a strong transactivator. BRCA1 protein possess a number of features common to transcriptional regulatory proteins, and also several lines of evidence suggests that it has a role in maintaining the genomic integrity. These suggests that BRCA1 may regulate the expression of one or more genes, which therefore represent potential effectors of BRCA1 tumor suppressor function. Although there are several studies about the function of BRCA1, the exact biochemical and tumor suppressor function of this gene remains to be identified. Our main research interest is to identify the genes targeted by BRCA1 gene. In this line we have a mammalian cell system where BRCA1 gene expression level can be controlled. By using differential gene expression method and making subtractive cDNA libraries we will try to answer some questions about the function of BRCA1 gene. We are also interested in identifying the genetic changes in different tumor tissue types which may be the cause of the tumor formation.