On this day 34 years ago, on June 16, 1991 (May 5, 1991 lunar calendar), human blood substitutes were made in the United States. On June 16, 1991, DNX Corporation of the United States announced that its scientists had successfully bred a pig with altered growth genes, and its blood carried human hemoglobin. The company will file a clinical trial application with the US authorities to determine the safety of this blood substitute into human beings. The Princeton, New Jersey-based company said that in order to breed this pig, they first removed embryos that had been inseminated for only one day, injected two sets of genes that could produce human hemoglobin, and then implanted the embryos into other pigs. Only three piglets obtained through this genetic engineering carried human hemoglobin. Scientists are currently using bacteria, yeast, insects and other mammals to create human blood substitutes. The main reason why DNX chose pigs is that their hemoglobin is similar to human hemoglobin, and it does not have side effects when genetically replaced. In addition, pigs are highly reproductive and grow rapidly.