dhfr activity assay?

[Shanthi Adimoolam]

Hi,
I am trying to overproduce some mutants in mammalian cells which are 
deficient in their normal activity. The selectable marker that i am using is the dhfr gene 
and i am expressing these mutants in CHO/dhfr- cells. I am wondering if 
there is an assay for dhfr (dihydrofolate reductase) that will enable me 
to select for colonies expressing high levels of the mutant protein. Any 
reference or some info would be extremely helpful.
Thanks in advance.
-shanthi