The invention is directed to a method of mutating one or more target nucleic acid sequences in a stem cell or a zygote comprising introducing into the stem cell or zygote (i) ribonucleic acid (RNA) sequences that comprise a portion that is complementary to a portion of each of the target nucleic acid sequences and comprise a binding site for a CRISPR associated (Cas) protein; and a Cas nucleic acid sequence or a variant thereof that encodes a Cas protein having nuclease activity. The stem cell or zygote is maintained under conditions in which the target nucleic acid sequences are mutated in the stem cell or zygote. The invention is also directed to methods of producing a non human mammal carrying mutations and methods of modulating the expression and/or activity target nucleic acid sequences and cells or zygotes.