An in vitro system was established for the quantitative chemical transformation of mouse epithelial cells mus musculus castaneous epithelial (MMCE). These cells can also be transformed by C3H/MuLV in conjunction with 12-0-tetradecanovlphorbol-13-acetate (TPA). Transformed cell clones obtained from soft agar generally were virus nonproducers. The purpose of these experiments is to sequence-label TPA-promotable cellular tumor genes and thus make possible their isolation by molecular cloning.