Real applications for society were first seen in 1990, when gene therapy experiments began on humans to treat incurable diseases. At this time, Scottish scientists have announced the birth of a lamb (Dolly) cloned by nuclear transfer using a mammary cell of an adult sheep. This result demonstrates that for the first time humanity has been able to clone a complex organism in the laboratory. Following this realization, several mammals were cloned and numerous human genes were transferred to rats, monkeys and dogs creating the humanized animals. This type of animals can be used in medical research with a model to study human diseases