We use CRISPR/CAS9 to induce site-specific mutations in genes, putative regulatory elements, and SNPs in the human and mouse genomes. CRISPR stands for "clustered regularly interspaced short palindromic repeats," which in combination with the prokaryotic RNA-directed DNA nuclease CAS9 acts as an adaptive bacterial immune system against DNA bacteriophages. In bacteria, complementary CRISPR RNAs (crRNA and tracrRNA) are transcribed and form an RNA duplex. This duplex is cleaved and subsequently acts to guide CAS9 to viral DNA sequences that are complementary to the crRNA. Recently, CRISPR/CAS9 has been re-engineered into eukaryotic transient and stable expression systems as a powerful genome editing tool for eukaryotic cells. Importantly, we are applying this powerful approach to both human cells in vitro, and for generating transgenic mice that lack evolutionarily conserved regulatory elements to create new models to understand genetic disease susceptibility at an organismal level.