GpIb’s growth-inhibitory effect maps to its 14-3-3 interaction domain. In (A), cell number was measured 4 days after replating. Truncation of 16 C-terminal amino acids of GpIb (594/β/IX), a region reported to represent the binding site for 14-3-3ζ, restores cell number to control (vector) levels. A second mutation eliminating the actin binding domain, but preserving the terminal 14-3-3 binding domain of GpIb (del/β/IX), shows decreased cell number comparable to CHO/β/IX cells. In (B), propidium-labeled cells were analyzed for DNA content. CHO cells expressing 594/β/IX enter S-phase normally, but those expressing del/β/IX are arrested in G1 (n = 3).