Cell number is decreased after passage of CHO cells expressing GpIb. Single cells were cloned by limiting dilution and the quantity of GpIb expressed in each cloned cell line was checked by flow cytometry as in Fig 1. (A) shows the stable expression of GpIb in vector (V/β/IX), low (/β/IX-L), and high (/β/IX-H) expressing cloned cell lines. Three cell lines with high expression of GpIb and three cell lines with low expression of GpIb, as defined by immunoblot analysis of total GpIb  (B), were cultured on a vWF substrate and the cell number was counted 4 days after passage. (C) demonstrates that the cell number is decreased in proportion to the quantity of GpIb expressed by that cell line. (**P < .0001 in comparison with V/β/IX cells; n = 3.)