There are many types of CHO (Chinese Hamster Ovary) cells in use today. It is a bit of a mystery when CHO cells originated but it is thought the line was derived in the 1960's. Dr. Theodore Puck received a female Chinese Hamster from a laboratory at the Boston Cancer Research Foundation and used the animal to derive the original cell lines. One line was known as CHO-K1, was obtained from those first CHO lines. CHO-K1 is an adherent line and was traditionally grown of F12 Medium supplemented with 10% FBS. The CHO lines were thought to be important because they had few chromosomes for a mammalian cell and were used for radiation cytogenetics and they were known for their ease of culturing. CHO-K1 cells also do not express EGFR.
CHO-DG44 cells were derived from the original CHO-K1 cells by several rounds of mutagenesis that deleted both copies of the dhfr genes. Dihydrofolate reductase (dhfr) is required for the de novo synthesis of purines, thymidylic acid, and certain amino acids and is required for growth and proliferation of CHO cells. As a result CHO-DG44 cells require glycine, hypoxanthine and thymidine (GHT) for growth. This requirement allows for selection of recombinant clones by transfection of recombinant DNA plasmids containing a gene of interest and the replacement dhfr gene if cells grow on GHT minus medium then they have taken up the recombinant plasmid DNA and hopefully the gene of interest as well.