we now report that our procedure -- co-transfection with the chicken thymidine kinase gene ( tk ) and whole cellular dna , selection with hypoxanthine-aminopterin-thymidine ( hat ) , and staining of the cells with fluorochrome-conjugated monoclonal antibodies and fluorescence-activated cell-sorter ( facs ) selection -- yields transfectants expressing a variety of cell-surface molecules ( 19 of 21 investigated ) , most at a frequency of about one per 10 ( 3 ) tk+ transformants .
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