1. To remove attached cells, briefly rinse the cell layer with 0.25% (w/v) Trypsin-0.03% (w/v) EDTA solution to remove all traces of serum which contains trypsin inhibitor.
2. Briefly rinse the cell layer with 0.25% (w/v) Trypsin-0.53mM EDTA solution to remove all traces of serum which contains trypsin inhibitor.
3. Add 2.0 to 3.0 mL of Trypsin-EDTA solution to flask and observe cells under an inverted microscope until cell layer is dispersed (usually within 5 to 15 minutes).
   Note: To avoid clumping do not agitate the cells by hitting or shaking the flask while waiting for the cells to detach. Cells that are difficult to detach may be placed at 37°C to facilitate dispersal.
4. Add 6.0 to 8.0 mL of complete growth medium and aspirate cells by gently pipetting.
5. Add appropriate aliquots of the cell suspension to new culture vessels.
6. Incubate cultures at 37°C.

Subcultivation Ratio: 1:4 to 1:12
Medium Renewal: Every 2 to 3 days

Note: For more information on enzymatic dissociation and subculturing of cell lines consult Chapter 10 in Culture of Animal Cells, a Manual of Basic Technique by R. Ian Freshney, 3rd edition, published by Alan R. Liss, N.Y., 1994.

Complete growth medium described above supplemented with 5% (v/v) DMSO.xa0 Cell culture tested DMSO is available as ATCC Catalog No. 4-X.

Thompson LH, et al. Repair of DNA adducts in asynchronous CHO cells and the role of repair in cell killing and mutation induction in synchronous cells treated with 7-bromomethylbenzaanthracene. Somatic Cell Mol. Genet. 10: 183-194, 1984. PubMed: 6584989

Thompson LH, et al. Genetic diversity of UV-sensitive DNA repair mutants of Chinese hamster ovary cells. Proc. Natl. Acad. Sci. USA 78: 3734-3737, 1981. PubMed: 6943579

Hoy CA, et al. Defective DNA cross-link removal in Chinese hamster cell mutants hypersensitive to bifunctional alkylating agents. Cancer Res. 45: 1737-1743, 1985. PubMed: 3919945

Busch D, et al. Summary of complementation groups of UV-sensitive CHO cell mutants isolated by large-scale screening. Mutagenesis 4: 349-354, 1989. PubMed: 2687628

Bessho T, et al. Initiation of DNA interstrand cross-link repair in humans: the nucleotide excision repair system makes dual incisions 5" to the cross-linked base and removes a 22- to 28-nucleotide-long damage-free strand. Mol. Cell. Biol. 17: 6822-6830, 1997. PubMed: 9372913

Thompson LH, et al. Hypersensitivity to mutation and sister-chromatid-exchange induction in CHO cell mutants defective in incising DNA containing UV lesions. Somatic Cell Genet. 8: 759-773, 1982. PubMed: 7163954

Thompson LH, et al. A screening method for isolating DNA repair-deficient mutants of CHO cells. Somatic Cell Genet. 6: 391-405, 1980. PubMed: 7404270

Hay, R. J., Caputo, J. L., and Macy, M. L., Eds. (1992), ATCC Quality Control Methods for Cell Lines. 2nd edition, Published by ATCC.

Caputo, J. L., Biosafety procedures in cell culture. J. Tissue Culture Methods 11:223-227, 1988.

Fleming, D.O., Richardson, J. H., Tulis, J.J. and Vesley, D., (1995) Laboratory Safety: Principles and Practice. Second edition, ASM press, Washington, DC.

Biosafety in Microbiological and Biomedical Laboratories, 5th ed. HHS. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Washington DC: U.S. Government Printing Office; 2007. The entire text is available online.