That is an old vector. I don't like this vector. But you can use another
expression vector for this purpose. Check Invitrogen's catalogue.
Anyhow, you can find your answer about pCDM8 vector expression
screening. Here are the authors who originally developed it.
Aruffo A, Seed B
Molecular cloning of a CD28 cDNA by a high-efficiency COS cell
Proc Natl Acad Sci U S A 1987 84(23) 8573-7.
n.kaushik at ic.ac.uk