Checkerboard analysis of AS283 motility response to Tat-MBP. Different gradient conditions were obtained by adding the indicated concentrations of Tat-MBP to the upper and lower compartments of the Boyden chamber. Cells, in the upper compartment, were exposed to conditions of null gradient (same concentration of Tat-MBP in the upper and lower compartments, diagonal), positive gradients (higher concentrations of Tat-MBP in the lower compartment, below the diagonal), or negative gradients (higher concentrations of Tat-MBP in the upper compartment, above the diagonal). Data are expressed as the number of migrated cells in 20 high-power fields (mean and SD of triplicates).