Flow cytometric analysis of mutant GP Ibα surface expression in transiently transfected CHO cells. CHO βIX cells were transiently transfected with plasmids containing a wild-type GP Ibα cDNA or cDNAs containing the AT deletion, alone and in combination with the 5′ mutation (see Fig 4). Both mutants produced lower surface levels of GP Ibα than did the wild-type plasmid (Student's t-test, n = 5, P < .0001), but were not significantly different from each other (P = .53). Surface fluorescence in sham-transfected cells was significantly lower than in cells transfected with the AT-deletion mutant (P < .05).