Abstract: The present invention generally relates to therapeutic inhibition of protein arginine methyltransferase 5 (PRMT5). In particular, cell lines having MTAP loss and increased intracellular MTA concentrations show selective dependence on PRMT5. Thus, the invention also relates to methods of identifying and treating PRMT5-related diseases in subjects or tissues which have an MTAP deficiency, alone or in combination, with a second agent that reduces MTAP activity and/or increases intracellular MTA levels, and/or provides an MTA analogs to the cell or tissue. The invention also relates to the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas System and components thereof. More specifically, the present invention relates to the delivery, use and therapeutic applications of the CRISPR-Cas systems and compositions in tumor cells ex vivo and/or in vivo. For example using methods disclosed herein, cells can be sensitized to PRMT5 inhibition.