sentence: By designing two zinc finger motifs recognizing either side of 5 – 6 bp spacer sequences at a target region , FokI nuclease combined with zinc finger can introduce DSBs within a target region ( Kim et al ., 1996 ; Smith et al ., 1999 ; Bibikova et al ., 2003 ; Porteus and Baltimore , 2003 ; Urnov et al ., 2010 ) .