sentence: As early as 2002 , pigs with heterozygously knockout GGTA1 were produced using HR ( Dai et al ., 2002 ; Lai et al ., 2002 ) , and homozygous GGTA1 knockout ( GTKO ) piglets were born by further screening of homozygous knockout cells from the heterozygous pigs ( Phelps et al ., 2003 ; Kolber-Simonds et al ., 2014 ) .