The inhibitory effect of GpIb on cell growth is specifically dependent on the expression of GpIb and is mediated by its cytoplasmic domain. Expression of GpIb with muristerone (M) was induced in CHO In/β/IX cells grown on vWF. Four days after the induction of GpIb with muristerone (upper part of [A]), the In/β/IX CHO cells are reduced in number (lower part of [B]). Cell number was also measured 4 days after replating in CHO cells transduced with a truncated GpIb (at aa 540, which eliminates its actin binding protein and 14-3-3 adapter protein interactions). The mutant GpIb is expressed on the cell surface at levels comparable to the wild-type (A), but this truncation eliminates the growth-inhibitory effect of wild-type GpIb. (**P < .0001 compared with V/β/IX; *P < .001 compared with V/β/IX; n = 3.)