The high MF of these clones of DLD1 is due to the defect in the MSH6 MMR gene of this cell line, and similarly, high MF of DLD1 or other MSI colon cancer cells compared with normal diploid fibroblasts has been reported elsewhere using the HPRT or ouabainR endogenously expressed gene systems (Bhattacharyya et al, 1994, 1995). Here, HPRT1 is linked to malignant colon neoplasm.