Occasional changes in the genetic makeup of cultured cells allow them to proliferate indefinitely, making them effectively immortal. Such lines are said to be transformed, and are often used in research as a standardized cell line. Chinese hamster ovary (CHO) cells were introduced in the early 1960s as a viable epithelial cell line containing twin female X chromosomes. The most common type of CHO cells has a nutritional requirement for the amino acid proline, which makes this cell line an ideal candidate for genetic studies.