Cancer is a highly heterogeneous disease in term of molecular signature even though it is originated from the same tissue type.Cancer heterogeneity may occur during its development or treatment,which is the main cause resulting in drug resistance and recurrence.Precision medicine refers to matching the right medicine to the right patients based on their molecular signatures.Therefore,a thorough understanding of the mechanism of tumorigenesis and drug resistance is essential to precision medicine.CRISPR-cas9 system is a powerful tool for gene editing and CRISPR-based high-throughput screening has been widely applied especially in searching for tumor-driven or synergistic lethal genes aiming to overcome drug resistance.In this review,we describethe progress of CRlSPR-cas9-based unbiased screening in precision medicine including identification of new drug targets,biomarkers and elucidation of mechanisms leading to drug resistance.The existing challenges as well as the future directions are also discussed.