The siRNA technology is a promising research tool for use in functional genomics, and it also shows potential for use in future therapy. Thus far, siRNAs have been used to specifically silence disease-associated alleles in animal models. The technology is still quite new, and the biological mechanisms underlying RNAi and siRNA-mediated knockdown of gene expression are not yet fully understood. The main issues when siRNAs are designed are efficiency and specificity, and it is of great importance to consider possible off-target effects in the siRNA design. One major challenge in siRNA-based therapy is the development of systems for efficient delivery to the target cells. A large number of tools have, over the last few years, been designed for the delivery of DNA and RNA for gene therapy, and extensive efforts are now placed into developing clinical applications of siRNAs in a range of human diseases, including breast cancer.