The older the donated nucleus is, the harder it is to make a clone. Fetal cells are harder to clone than embryonic cells, and adult cells are harder than fetal cells. The same team of scientists also tried to clone macaques using nuclei from adult monkeys, but it did not work. They implanted 42 mother monkeys, and 22 of them became pregnant, but there were only two infant macaques, and they died soon after birth.