We present Cas-Designer, a user-friendly program to aid researchers in choosing appropriate target sites in a gene of interest for type II CRISPR/Cas-derived RNA-guided endonucleases, which are now widely used for biomedical research and biotechnology. Cas-Designer rapidly provides the list of all possible guide RNA sequences in a given input DNA sequence and their potential off-target sites including bulge-type sites in a genome of choice. In addition, the program assigns an out-of-frame score to each target site to help users choose appropriate sites for gene knockout. Cas-Designer shows the results in an interactive table and provides user-friendly filter functions.