Scientists made the first cloned mammal, Dolly the sheep, in 1996 using somatic cell nuclear transfer (SCNT). Since then, scientists have cloned 23 different kinds of mammals, including cows, cats, dogs, horses, and rats. But primates have been very hard to clone. Reprogramming transferred nuclei is very difficult. A monkey named Tetra (born October 1999), a female rhesus macaque, was made by a team led by Gerald Schatten of the Oregon National Primate Research Center using a different technique, called "embryo splitting," which is a way of making an artificial twin. Embryo splitting is not as hard as somatic cell nuclear transfer, and it can only be done from an embryo. So some people say Tetra is the first cloned primate and other people say Zhong Zhong and Hua Hua are.