Different protospacers have different efficiency for producing mutations. In other words not every spacer in front of the PAM (protospacer adjacent motif) will be cut with the same efficiency as every other spacer. Software like CHOPCHOP can help you to design gene editing experiments with a much high rate of success.
Important considerations
- You want your gRNA to cut your target with high efficiency but not to cut other parts of the genome. Software helps for this.
- Mismatches in the seed region of the target sequence are more likely to prevent your gRNA from cutting than mismatches further from the PAM.