Chinese Hamster Ovary cells (CHO) have been around a long time, since the 1960s and as with most of the early-cultured cells they are derived from a rodent. Rodent cells were used to create the first homogeneous cell lines and media formulations. Many events occurred to bring CHO cells to the forefront in biotechnology. CHO cells became popular because they have a low chromosome number and were used to study genetics. As a result CHO cells have been grown for a long time and much is known about their genetics and growth characteristics. Additionally CHO cells so not express the EGR receptor so the cell line was used in reconstitution studies to demonstrate the structure/function of the receptor. As time went on, CHO cells were selected for recombinant protein production because they grew well in culture and some of the CHO lines had selection capabilities for cells containing recombinant DNA plasmids. The next events that catapulted CHO cells into prominence was a combination of FDA approval for bio-therapeutic protein production, the commercial production of animal origin free media developed specifically for a single cell type such as CD-CHO medium and the cells ability to grow in suspension. These characteristics have made CHO cells the most important cell line for the production of therapeutic proteins including humanized antibodies.