Increasing specificity for RNA-guided genome editing

A method of inducing a single or double-stranded break in a target region of a double-stranded DNA molecule the method comprising expressing in or introducing into the cell:

• a Cas9 nuclease or nickase;
• a tracrRNA;
• and
• a crRNA that includes one or more nucleotides that are modified,
• wherein the ribose phosphate backbone has been replaced by a polyamide chain,
• wherein the crRNA consists of the sequence:
• (SEQ ID NO:
• 5)
• (X17-20)GUUUUAGAGCUA;
• (SEQ ID NO:
• 6)
• (X17-20)GUUUUAGAGCUAUGCUGUUUUG;
• or
• (SEQ ID NO:
• 7)
• (X17-20)GUUUUAGAGCUAUGCU,
• wherein X17-20 is a sequence of 17-20 nucleotides that are complementary to the target sequence, preferably the target sequence immediately 5′ of a protospacer adjacent motif (PAM).