Complex sentence: On subsequent invasion, Cas9 nuclease attached to tracrRNA: crRNA is guided to the invading protospacer sequence, but Cas9 will not cleave the protospacer sequence unless there is an adjacent PAM sequence.
Split into simpler sentences: On subsequent invasion, Cas9 nuclease attaches to tracrRNA: crRNA which guides Cas9 to the invading protospacer sequence. But Cas9 will not cleave the protospacer sequence unless there is an adjacent PAM sequence.