The first attempts to use animal organs in people date from 1905. In 1992, there were two unsuccessful attempts to transplant baboon livers into humans. The first transgenic pigs were created in 1993. Human-immune-system genes were inserted into the pig embryos and so they develop with organs that contain human complement chemicals. In theory, this means that the human immune system should be unable to distinguish these organs from human ones, and that the pig's organs would be capable of being transplanted into humans without tissue rejection. Medical specialists are concerned about cross-species disease transmission as a side-effect of the procedure.