As others have pointed out, there is no single "mutated gene sequence responsible for breast cancer".
If you're looking for SNPs that increase susceptibility to the disease, a good starting point is OMIM. You can search with a simple term (e.g. "breast cancer") to get started. Then, click on the tab that says "OMIM dbSNP" to limit results to those loci with a known SNP. View a gene from the list of results and you'll see a "SNP" link in the links box on the right of the page, which will take you to dbSNP entries - e.g.this page for BRCA1.
You can also do all of this programmatically, but I'd learn the basics of how the website works first.
Try the International Cancer Genome Consortium site. The data is new and still flowing in, but worth checking. The data portal is http://dcc.icgc.org/ , but for a grasp of the whole project see the main site: http://icgc.org/