sentence: Based on the GTKO pigs , numerous other genetic modifications were performed to further overcome xenogeneic barriers ; these approaches mainly include individual or combined overexpression of transgenes , such as CD46 , CD55 , CD59 , CD39 , thrombomodulin , heme oxygenase 1 , A20 , HLA-E , and CD47 , to prevent complement activity , delayed xenograft rejection , or / and acute cellular rejection ( Fischer et al ., 2016 ; Hryhorowicz et al ., 2017 ; Laird et al ., 2017 ) .