Abstract: The present invention relates to novel strains of pig that are highly suitable for xenotransplantation. The first novel pig strain lacks functional porcine endogenous retroviruses so is suitable as a donor for tissue and/or cell xenotransplantation into a human recipient. These pigs can also be used as a foundation pig for further manipulation, for example, by gene editing of xenoantigens to produce a second novel strain of pig that is not only free of infectious porcine retroviruses but is also free of the main xenoantigens responsible for hyperacute organ rejection. These pigs can be used for whole organ, tissue and/or cell transplantation into a human recipient.