The article describes the development and application of CRISPR/Cas9 systems for precise genome engineering in mammalian cells. The authors engineered two type II CRISPR/Cas9 systems, demonstrating that Cas9 nucleases can be directed by short RNAs to induce specific DNA cleavage at endogenous genomic loci in human and mouse cells. They also showed that Cas9 can be converted into a nicking enzyme to facilitate homology-directed repair with minimal mutagenic activity. Additionally, multiple guide sequences can be encoded into a single CRISPR array to enable simultaneous editing of several sites within the genome. The study highlights the ease of programmability and wide applicability of the RNA-guided nuclease technology, making it a powerful tool for systematic reverse engineering of causal genetic variations.The article describes the development and application of CRISPR/Cas9 systems for precise genome engineering in mammalian cells. The authors engineered two type II CRISPR/Cas9 systems, demonstrating that Cas9 nucleases can be directed by short RNAs to induce specific DNA cleavage at endogenous genomic loci in human and mouse cells. They also showed that Cas9 can be converted into a nicking enzyme to facilitate homology-directed repair with minimal mutagenic activity. Additionally, multiple guide sequences can be encoded into a single CRISPR array to enable simultaneous editing of several sites within the genome. The study highlights the ease of programmability and wide applicability of the RNA-guided nuclease technology, making it a powerful tool for systematic reverse engineering of causal genetic variations.