Examination by confocal fluorescence microscopy of cultures established from CEACAM1-4S positive cells isolated by FACS showed intense membrane fluorescence when labeled by indirect immunofluorescence with CEACAM1 specific MAb 9.2, confirming that both wild type and mutant forms of CEACAM1-4S were properly transported to the plasma membrane ( Figure 2 , A-L). Empty vector transfected and untransfected 253-NT cells showed no detectable reactivity with MAb 9.2 ( Figure 2 , E and J, respectively). Immunoblot analysis indicated that the wild type and mutant proteins all had an apparent molecular mass of 105 kDa, the expected size for CEACAM1-4S isoform [2] , suggesting that post-translational processing has not been altered ( Figure 3 ). Quantitative analysis by flow cytometry of FACS selected, stably transfected cells labeled by IIF with MAb 9.2 indicated that 72-83% of the cells were positive for either wild type or mutated forms of CEACAM1-4S (Figure 4 ).