DNA repair systems function as the last resort to permanent DNA mutation. Specific genes correspond to specific repair systems: MPG is involved in base excision repair (BER); XPA is vital for nucleotide excision repair (NER); and KU-80 is necessary for double-stranded break repair (DSBR). Creating small interfering RNA (siRNA) constructs that target specific mRNAs down regulates expression of DNA repair system genes that could lead to changes in cellular phenotype. Transient transfection permits only temporary reduction in expression. The goal of my project is to find a method whereby stable transfection into the genomic DNA could occur with permanent reduction of expression of MPG, KU-80, and XPA that would lead to a manifestation of distinct phenotypes.