During the 1990s, I was studying electroporation of various cell types (plant and human origin). I have not tested CHO cells specificall, but two chapters (Chapter 8 and Chapter 19) in the book of Nickoloff, which in my opinion is one the best reference in this area, are dedicated to CHO cells The book is J.A. Nickoloff(Ed), Animal Cell Electroporation and Electrofusion Protocols in Methods in Molecular Biology) Vol. 48 (1995) Humana Press,Totowa, New Jersey.