CHO-K1 are dihydrofolate reductase (DHFR) positive. There are other mutants made from CHO-K1 that are DHFR deficient. Please see the links below to review these possible options:
CHO/dhFr-
https://www.sigmaaldrich.com/product/sigma/cb_94060607
CHO/dhFr-AC-free
https://www.sigmaaldrich.com/product/sigma/cb_05011002
CHOZN® ZFN-Modified CHO
https://www.sigmaaldrich.com/product/sigma/chodhfr