sentence: In addition , corresponding knockout pigs with double ( GGTA1 / CMAH ) and triple ( GGTA1 / CMAH / B4GALNT2 ) gene inactivation were established ( Lutz et al ., 2013 ; Estrada et al ., 2015 ; Miyagawa et al ., 2015 ; Gao et al ., 2017 ) .